tests/gnrc_sixlowpan: fix stacksize of dummy device
tests/gnrc_sixlowpan: fix stacksize of dummy device

This was already too small in 6c2510dc so it should have been properly
tested. Now that we are testing it on CI with binaries compiled by LLVM
this leads to crashes with this "new" platform.

cladmi/RIOT,x3ro/RIOT,yogo1212/RIOT,kbumsik/RIOT,RIOT-OS/RIOT,basilfx/RIOT,gebart/RIOT,toonst/RIOT,lazytech-org/RIOT,biboc/RIOT,mtausig/RIOT,ant9000/RIOT,avmelnikoff/RIOT,aeneby/RIOT,kbumsik/RIOT,biboc/RIOT,smlng/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kYc0o/RIOT,BytesGalore/RIOT,mtausig/RIOT,gebart/RIOT,lazytech-org/RIOT,yogo1212/RIOT,x3ro/RIOT,miri64/RIOT,x3ro/RIOT,basilfx/RIOT,rfuentess/RIOT,BytesGalore/RIOT,authmillenon/RIOT,kbumsik/RIOT,kYc0o/RIOT,yogo1212/RIOT,josephnoir/RIOT,smlng/RIOT,x3ro/RIOT,toonst/RIOT,rfuentess/RIOT,avmelnikoff/RIOT,BytesGalore/RIOT,mfrey/RIOT,cladmi/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,smlng/RIOT,kbumsik/RIOT,cladmi/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,ant9000/RIOT,yogo1212/RIOT,kYc0o/RIOT,josephnoir/RIOT,yogo1212/RIOT,OlegHahm/RIOT,miri64/RIOT,A-Paul/RIOT,OlegHahm/RIOT,gebart/RIOT,aeneby/RIOT,RIOT-OS/RIOT,lazytech-org/RIOT,authmillenon/RIOT,gebart/RIOT,authmillenon/RIOT,aeneby/RIOT,jasonatran/RIOT,biboc/RIOT,A-Paul/RIOT,jasonatran/RIOT,miri64/RIOT,aeneby/RIOT,ant9000/RIOT,josephnoir/RIOT,mtausig/RIOT,toonst/RIOT,A-Paul/RIOT,BytesGalore/RIOT,kaspar030/RIOT,kaspar030/RIOT,A-Paul/RIOT,avmelnikoff/RIOT,x3ro/RIOT,jasonatran/RIOT,josephnoir/RIOT,kbumsik/RIOT,smlng/RIOT,authmillenon/RIOT,OlegHahm/RIOT,toonst/RIOT,josephnoir/RIOT,basilfx/RIOT,kYc0o/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,lazytech-org/RIOT,mtausig/RIOT,kYc0o/RIOT,mfrey/RIOT,mfrey/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,basilfx/RIOT,toonst/RIOT,cladmi/RIOT,avmelnikoff/RIOT,rfuentess/RIOT,mfrey/RIOT,BytesGalore/RIOT,authmillenon/RIOT,biboc/RIOT,rfuentess/RIOT,OTAkeys/RIOT,basilfx/RIOT,avmelnikoff/RIOT,smlng/RIOT,kaspar030/RIOT,ant9000/RIOT,gebart/RIOT,A-Paul/RIOT,miri64/RIOT,mtausig/RIOT,miri64/RIOT,mfrey/RIOT,kaspar030/RIOT,OlegHahm/RIOT,cladmi/RIOT,RIOT-OS/RIOT,biboc/RIOT,aeneby/RIOT,rfuentess/RIOT,authmillenon/RIOT