unittests: make: Fix unittests for branch changes
unittests: make: Fix unittests for branch changes

Changing between branches in git typically leaves directories empty and
does not remove them. If you have different unittests in different
branches this leads to an error in building them, since the current
configuration expects a Makefile in all `tests-*` directory. This change
configures Make to search for a Makefile in `tests-*` rather than just
assuming that there is one.

JensErdmann/RIOT,wentaoshang/RIOT,koenning/RIOT,binarylemon/RIOT,jfischer-phytec-iot/RIOT,JensErdmann/RIOT,LudwigOrtmann/RIOT,altairpearl/RIOT,wentaoshang/RIOT,RubikonAlpha/RIOT,backenklee/RIOT,immesys/RiSyn,sumanpanchal/RIOT,benoit-canet/RIOT,gbarnett/RIOT,kbumsik/RIOT,marcosalm/RIOT,watr-li/RIOT,tdautc19841202/RIOT,kb2ma/RIOT,kushalsingh007/RIOT,hamilton-mote/RIOT-OS,alex1818/RIOT,biboc/RIOT,kaspar030/RIOT,MonsterCode8000/RIOT,Hyungsin/RIOT-OS,altairpearl/RIOT,Darredevil/RIOT,MohmadAyman/RIOT,herrfz/RIOT,phiros/RIOT,jferreir/RIOT,shady33/RIOT,immesys/RiSyn,RubikonAlpha/RIOT,herrfz/RIOT,brettswann/RIOT,mziegert/RIOT,d00616/RIOT,tfar/RIOT,adjih/RIOT,jasonatran/RIOT,beurdouche/RIOT,openkosmosorg/RIOT,lebrush/RIOT,Lexandro92/RIOT-CoAP,ks156/RIOT,immesys/RiSyn,khhhh/RIOT,Ell-i/RIOT,attdona/RIOT,ThanhVic/RIOT,A-Paul/RIOT,rajma996/RIOT,Osblouf/RIOT,d00616/RIOT,arvindpdmn/RIOT,phiros/RIOT,centurysys/RIOT,JensErdmann/RIOT,Darredevil/RIOT,dailab/RIOT,josephnoir/RIOT,nsol-nmsu/RIOT,kerneltask/RIOT,altairpearl/RIOT,OTAkeys/RIOT,roberthartung/RIOT,kushalsingh007/RIOT,neiljay/RIOT,backenklee/RIOT,alignan/RIOT,kerneltask/RIOT,aeneby/RIOT,phiros/RIOT,gbarnett/RIOT,kb2ma/RIOT,OlegHahm/RIOT,rousselk/RIOT,d00616/RIOT,dailab/RIOT,BytesGalore/RIOT,koenning/RIOT,adjih/RIOT,backenklee/RIOT,rousselk/RIOT,robixnai/RIOT,immesys/RiSyn,backenklee/RIOT,dkm/RIOT,locicontrols/RIOT,syin2/RIOT,ThanhVic/RIOT,wentaoshang/RIOT,immesys/RiSyn,jasonatran/RIOT,gautric/RIOT,arvindpdmn/RIOT,kushalsingh007/RIOT,1blankz7/RIOT,toonst/RIOT,koenning/RIOT,centurysys/RIOT,robixnai/RIOT,abkam07/RIOT,dkm/RIOT,Osblouf/RIOT,yogo1212/RIOT,beurdouche/RIOT,stevenj/RIOT,asanka-code/RIOT,fnack/RIOT,ant9000/RIOT,lebrush/RIOT,Osblouf/RIOT,centurysys/RIOT,sumanpanchal/RIOT,altairpearl/RIOT,l3nko/RIOT,asanka-code/RIOT,ximus/RIOT,dkm/RIOT,Josar/RIOT,Lotterleben/RIOT,mziegert/RIOT,centurysys/RIOT,LudwigOrtmann/RIOT,kerneltask/RIOT,khhhh/RIOT,beurdouche/RIOT,Lexandro92/RIOT-CoAP,chris-wood/RIOT,immesys/RiSyn,Yonezawa-T2/RIOT,tdautc19841202/RIOT,rfswarm2/RIOT,gbarnett/RIOT,rousselk/RIOT,MohmadAyman/RIOT,bartfaizoltan/RIOT,Yonezawa-T2/RIOT,adrianghc/RIOT,mtausig/RIOT,aeneby/RIOT,rfswarm/RIOT,rfuentess/RIOT,miri64/RIOT,marcosalm/RIOT,Yonezawa-T2/RIOT,attdona/RIOT,TobiasFredersdorf/RIOT,EmuxEvans/RIOT,MohmadAyman/RIOT,tdautc19841202/RIOT,rakendrathapa/RIOT,x3ro/RIOT,emmanuelsearch/RIOT,RIOT-OS/RIOT,ant9000/RIOT,Darredevil/RIOT,MohmadAyman/RIOT,FrancescoErmini/RIOT,herrfz/RIOT,patkan/RIOT,neumodisch/RIOT,koenning/RIOT,thiagohd/RIOT,jferreir/RIOT,ximus/RIOT,attdona/RIOT,jhollister/RIOT,phiros/RIOT,herrfz/RIOT,shady33/RIOT,rfuentess/RIOT,openkosmosorg/RIOT,authmillenon/RIOT,MarkXYang/RIOT,1blankz7/RIOT,luciotorre/RIOT,watr-li/RIOT,kaspar030/RIOT,automote/RIOT,MonsterCode8000/RIOT,abp719/RIOT,lebrush/RIOT,jferreir/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,mtausig/RIOT,kYc0o/RIOT,lebrush/RIOT,FrancescoErmini/RIOT,daniel-k/RIOT,patkan/RIOT,authmillenon/RIOT,attdona/RIOT,gebart/RIOT,alex1818/RIOT,plushvoxel/RIOT,jfischer-phytec-iot/RIOT,abp719/RIOT,locicontrols/RIOT,Lotterleben/RIOT,luciotorre/RIOT,mziegert/RIOT,rfswarm/RIOT,d00616/RIOT,arvindpdmn/RIOT,cladmi/RIOT,gebart/RIOT,abp719/RIOT,luciotorre/RIOT,DipSwitch/RIOT,binarylemon/RIOT,fnack/RIOT,benoit-canet/RIOT,kushalsingh007/RIOT,ThanhVic/RIOT,aeneby/RIOT,PSHIVANI/Riot-Code,ximus/RIOT,locicontrols/RIOT,jasonatran/RIOT,robixnai/RIOT,changbiao/RIOT,DipSwitch/RIOT,alex1818/RIOT,alignan/RIOT,rfuentess/RIOT,centurysys/RIOT,lebrush/RIOT,ThanhVic/RIOT,RBartz/RIOT,zhuoshuguo/RIOT,koenning/RIOT,x3ro/RIOT,smlng/RIOT,luciotorre/RIOT,jbeyerstedt/RIOT-OTA-update,ntrtrung/RIOT,phiros/RIOT,mtausig/RIOT,josephnoir/RIOT,kYc0o/RIOT,DipSwitch/RIOT,ntrtrung/RIOT,rajma996/RIOT,watr-li/RIOT,OTAkeys/RIOT,yogo1212/RIOT,RBartz/RIOT,sumanpanchal/RIOT,patkan/RIOT,smlng/RIOT,herrfz/RIOT-old,lebrush/RIOT,latsku/RIOT,jferreir/RIOT,emmanuelsearch/RIOT,binarylemon/RIOT,gautric/RIOT,gebart/RIOT,jferreir/RIOT,msolters/RIOT,rfswarm/RIOT,x3ro/RIOT,d00616/RIOT,EmuxEvans/RIOT,Osblouf/RIOT,Hyungsin/RIOT-OS,miri64/RIOT,l3nko/RIOT,gebart/RIOT,herrfz/RIOT-old,patkan/RIOT,MarkXYang/RIOT,jhollister/RIOT,dailab/RIOT,rfswarm2/RIOT,shady33/RIOT,tdautc19841202/RIOT,tfar/RIOT,mziegert/RIOT,zhuoshuguo/RIOT,sgso/RIOT,Lexandro92/RIOT-CoAP,miri64/RIOT,haoyangyu/RIOT,kaleb-himes/RIOT,marcosalm/RIOT,EmuxEvans/RIOT,OlegHahm/RIOT,mtausig/RIOT,abkam07/RIOT,bartfaizoltan/RIOT,mfrey/RIOT,yogo1212/RIOT,MohmadAyman/RIOT,MarkXYang/RIOT,herrfz/RIOT-old,rajma996/RIOT,automote/RIOT,PSHIVANI/Riot-Code,Lotterleben/RIOT,khhhh/RIOT,avmelnikoff/RIOT,rfswarm/RIOT,shady33/RIOT,emmanuelsearch/RIOT,alex1818/RIOT,MarkXYang/RIOT,A-Paul/RIOT,1blankz7/RIOT,plushvoxel/RIOT,PSHIVANI/Riot-Code,jhollister/RIOT,asanka-code/RIOT,syin2/RIOT,1blankz7/RIOT,wentaoshang/RIOT,jremmert-phytec-iot/RIOT,mfrey/RIOT,kerneltask/RIOT,msolters/RIOT,thomaseichinger/RIOT,locicontrols/RIOT,cladmi/RIOT,mfrey/RIOT,LudwigKnuepfer/RIOT,kbumsik/RIOT,ks156/RIOT,robixnai/RIOT,daniel-k/RIOT,rfuentess/RIOT,brettswann/RIOT,gbarnett/RIOT,abp719/RIOT,watr-li/RIOT,dailab/RIOT,changbiao/RIOT,Josar/RIOT,DipSwitch/RIOT,abkam07/RIOT,A-Paul/RIOT,malosek/RIOT,attdona/RIOT,openkosmosorg/RIOT,sgso/RIOT,kerneltask/RIOT,mtausig/RIOT,thiagohd/RIOT,koenning/RIOT,hamilton-mote/RIOT-OS,bartfaizoltan/RIOT,abkam07/RIOT,marcosalm/RIOT,plushvoxel/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,miri64/RIOT,gautric/RIOT,dhruvvyas90/RIOT,jfischer-phytec-iot/RIOT,authmillenon/RIOT,toonst/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,RubikonAlpha/RIOT,l3nko/RIOT,zhuoshuguo/RIOT,Lotterleben/RIOT,ks156/RIOT,phiros/RIOT,BytesGalore/RIOT,benoit-canet/RIOT,neumodisch/RIOT,sgso/RIOT,katezilla/RIOT,stevenj/RIOT,aeneby/RIOT,TobiasFredersdorf/RIOT,thomaseichinger/RIOT,ntrtrung/RIOT,chris-wood/RIOT,DipSwitch/RIOT,luciotorre/RIOT,jhollister/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,avmelnikoff/RIOT,thiagohd/RIOT,roberthartung/RIOT,adrianghc/RIOT,RBartz/RIOT,dhruvvyas90/RIOT,kb2ma/RIOT,roberthartung/RIOT,abp719/RIOT,1blankz7/RIOT,beurdouche/RIOT,malosek/RIOT,dailab/RIOT,LudwigOrtmann/RIOT,AnonMall/RIOT,daniel-k/RIOT,sumanpanchal/RIOT,gautric/RIOT,stevenj/RIOT,BytesGalore/RIOT,neumodisch/RIOT,asanka-code/RIOT,kaspar030/RIOT,asanka-code/RIOT,l3nko/RIOT,binarylemon/RIOT,rfswarm2/RIOT,rfswarm2/RIOT,bartfaizoltan/RIOT,Ell-i/RIOT,rfswarm2/RIOT,rajma996/RIOT,LudwigKnuepfer/RIOT,adrianghc/RIOT,cladmi/RIOT,OTAkeys/RIOT,biboc/RIOT,Darredevil/RIOT,rousselk/RIOT,basilfx/RIOT,d00616/RIOT,smlng/RIOT,gautric/RIOT,Lexandro92/RIOT-CoAP,yogo1212/RIOT,jferreir/RIOT,toonst/RIOT,Hyungsin/RIOT-OS,syin2/RIOT,ximus/RIOT,Hyungsin/RIOT-OS,locicontrols/RIOT,malosek/RIOT,basilfx/RIOT,kb2ma/RIOT,mziegert/RIOT,mfrey/RIOT,benoit-canet/RIOT,emmanuelsearch/RIOT,basilfx/RIOT,abkam07/RIOT,rfswarm/RIOT,brettswann/RIOT,authmillenon/RIOT,authmillenon/RIOT,FrancescoErmini/RIOT,Yonezawa-T2/RIOT,beurdouche/RIOT,stevenj/RIOT,jhollister/RIOT,toonst/RIOT,latsku/RIOT,A-Paul/RIOT,TobiasFredersdorf/RIOT,MonsterCode8000/RIOT,binarylemon/RIOT,jremmert-phytec-iot/RIOT,RIOT-OS/RIOT,haoyangyu/RIOT,authmillenon/RIOT,lazytech-org/RIOT,EmuxEvans/RIOT,gbarnett/RIOT,openkosmosorg/RIOT,rousselk/RIOT,automote/RIOT,neumodisch/RIOT,tfar/RIOT,kb2ma/RIOT,rfswarm2/RIOT,brettswann/RIOT,haoyangyu/RIOT,alex1818/RIOT,herrfz/RIOT-old,ks156/RIOT,jasonatran/RIOT,jremmert-phytec-iot/RIOT,ntrtrung/RIOT,hamilton-mote/RIOT-OS,rfuentess/RIOT,kaspar030/RIOT,patkan/RIOT,josephnoir/RIOT,thiagohd/RIOT,msolters/RIOT,benoit-canet/RIOT,OlegHahm/RIOT,altairpearl/RIOT,1blankz7/RIOT,gbarnett/RIOT,PSHIVANI/Riot-Code,binarylemon/RIOT,fnack/RIOT,emmanuelsearch/RIOT,robixnai/RIOT,bartfaizoltan/RIOT,rajma996/RIOT,AnonMall/RIOT,LudwigOrtmann/RIOT,AnonMall/RIOT,nsol-nmsu/RIOT,LudwigKnuepfer/RIOT,JensErdmann/RIOT,sgso/RIOT,lazytech-org/RIOT,Lotterleben/RIOT,RubikonAlpha/RIOT,FrancescoErmini/RIOT,LudwigOrtmann/RIOT,Lotterleben/RIOT,RBartz/RIOT,haoyangyu/RIOT,EmuxEvans/RIOT,ximus/RIOT,latsku/RIOT,abkam07/RIOT,msolters/RIOT,Josar/RIOT,neiljay/RIOT,neiljay/RIOT,thiagohd/RIOT,katezilla/RIOT,AnonMall/RIOT,msolters/RIOT,ant9000/RIOT,latsku/RIOT,TobiasFredersdorf/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,dhruvvyas90/RIOT,toonst/RIOT,msolters/RIOT,biboc/RIOT,asanka-code/RIOT,RIOT-OS/RIOT,AnonMall/RIOT,jremmert-phytec-iot/RIOT,tdautc19841202/RIOT,stevenj/RIOT,dkm/RIOT,Ell-i/RIOT,smlng/RIOT,sgso/RIOT,chris-wood/RIOT,adrianghc/RIOT,FrancescoErmini/RIOT,rakendrathapa/RIOT,Osblouf/RIOT,alignan/RIOT,changbiao/RIOT,ThanhVic/RIOT,wentaoshang/RIOT,thiagohd/RIOT,alignan/RIOT,emmanuelsearch/RIOT,ntrtrung/RIOT,Lexandro92/RIOT-CoAP,kushalsingh007/RIOT,openkosmosorg/RIOT,MarkXYang/RIOT,brettswann/RIOT,MohmadAyman/RIOT,sumanpanchal/RIOT,RBartz/RIOT,ntrtrung/RIOT,roberthartung/RIOT,nsol-nmsu/RIOT,rousselk/RIOT,bartfaizoltan/RIOT,JensErdmann/RIOT,EmuxEvans/RIOT,biboc/RIOT,adjih/RIOT,sgso/RIOT,yogo1212/RIOT,Lotterleben/RIOT,kbumsik/RIOT,katezilla/RIOT,lazytech-org/RIOT,x3ro/RIOT,avmelnikoff/RIOT,RubikonAlpha/RIOT,mziegert/RIOT,latsku/RIOT,Ell-i/RIOT,Darredevil/RIOT,x3ro/RIOT,hamilton-mote/RIOT-OS,neiljay/RIOT,jremmert-phytec-iot/RIOT,herrfz/RIOT-old,watr-li/RIOT,ant9000/RIOT,arvindpdmn/RIOT,Darredevil/RIOT,jfischer-phytec-iot/RIOT,RubikonAlpha/RIOT,khhhh/RIOT,changbiao/RIOT,benoit-canet/RIOT,avmelnikoff/RIOT,OlegHahm/RIOT,patkan/RIOT,kaleb-himes/RIOT,Hyungsin/RIOT-OS,Ell-i/RIOT,Yonezawa-T2/RIOT,malosek/RIOT,josephnoir/RIOT,adrianghc/RIOT,MonsterCode8000/RIOT,Josar/RIOT,attdona/RIOT,plushvoxel/RIOT,LudwigKnuepfer/RIOT,tfar/RIOT,A-Paul/RIOT,rakendrathapa/RIOT,lazytech-org/RIOT,zhuoshuguo/RIOT,changbiao/RIOT,thomaseichinger/RIOT,kYc0o/RIOT,marcosalm/RIOT,dhruvvyas90/RIOT,fnack/RIOT,rakendrathapa/RIOT,l3nko/RIOT,alignan/RIOT,automote/RIOT,aeneby/RIOT,ThanhVic/RIOT,changbiao/RIOT,tfar/RIOT,OlegHahm/RIOT,LudwigOrtmann/RIOT,khhhh/RIOT,jfischer-phytec-iot/RIOT,gebart/RIOT,neiljay/RIOT,lazytech-org/RIOT,arvindpdmn/RIOT,FrancescoErmini/RIOT,rajma996/RIOT,jhollister/RIOT,Osblouf/RIOT,dhruvvyas90/RIOT,jremmert-phytec-iot/RIOT,MarkXYang/RIOT,kaleb-himes/RIOT,haoyangyu/RIOT,chris-wood/RIOT,OTAkeys/RIOT,locicontrols/RIOT,avmelnikoff/RIOT,JensErdmann/RIOT,ks156/RIOT,ant9000/RIOT,DipSwitch/RIOT,Yonezawa-T2/RIOT,smlng/RIOT,biboc/RIOT,kaleb-himes/RIOT,kbumsik/RIOT,robixnai/RIOT,kbumsik/RIOT,kushalsingh007/RIOT,hamilton-mote/RIOT-OS,katezilla/RIOT,altairpearl/RIOT,basilfx/RIOT,ximus/RIOT,daniel-k/RIOT,alex1818/RIOT,chris-wood/RIOT,nsol-nmsu/RIOT,cladmi/RIOT,kYc0o/RIOT,AnonMall/RIOT,daniel-k/RIOT,adjih/RIOT,marcosalm/RIOT,syin2/RIOT,locicontrols/RIOT,centurysys/RIOT,BytesGalore/RIOT,basilfx/RIOT,PSHIVANI/Riot-Code,watr-li/RIOT,khhhh/RIOT,adjih/RIOT,thomaseichinger/RIOT,jbeyerstedt/RIOT-OTA-update,openkosmosorg/RIOT,daniel-k/RIOT,fnack/RIOT,shady33/RIOT,sumanpanchal/RIOT,cladmi/RIOT,abp719/RIOT,wentaoshang/RIOT,PSHIVANI/Riot-Code,syin2/RIOT,OTAkeys/RIOT,luciotorre/RIOT,josephnoir/RIOT,katezilla/RIOT,arvindpdmn/RIOT,mfrey/RIOT,zhuoshuguo/RIOT,malosek/RIOT,roberthartung/RIOT,jbeyerstedt/RIOT-OTA-update,herrfz/RIOT,stevenj/RIOT,fnack/RIOT,Josar/RIOT,dkm/RIOT,brettswann/RIOT,herrfz/RIOT,kaleb-himes/RIOT,l3nko/RIOT,rakendrathapa/RIOT,LudwigKnuepfer/RIOT,Lexandro92/RIOT-CoAP,malosek/RIOT,neumodisch/RIOT,jbeyerstedt/RIOT-OTA-update,neumodisch/RIOT,backenklee/RIOT,BytesGalore/RIOT,thomaseichinger/RIOT,rakendrathapa/RIOT,tdautc19841202/RIOT,nsol-nmsu/RIOT,TobiasFredersdorf/RIOT,RBartz/RIOT,shady33/RIOT,kYc0o/RIOT,automote/RIOT,dhruvvyas90/RIOT,rfswarm/RIOT,jasonatran/RIOT,yogo1212/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,MonsterCode8000/RIOT,latsku/RIOT,MonsterCode8000/RIOT,chris-wood/RIOT,automote/RIOT,zhuoshuguo/RIOT,miri64/RIOT,jbeyerstedt/RIOT-OTA-update,plushvoxel/RIOT,RIOT-OS/RIOT,haoyangyu/RIOT