TestHelper: Show error on passing invalid tests
TestHelper: Show error on passing invalid tests

TestHelper shows a "Test does not exist" error when invalid tests
are passed to ./run_tests.py script.

Fixes: https://github.com/coala-analyzer/coala/issues/384

arafsheikh/coala,swatilodha/coala,aptrishu/coala,arjunsinghy96/coala,RJ722/coala,damngamerz/coala,SambitAcharya/coala,meetmangukiya/coala,swatilodha/coala,Tanmay28/coala,mr-karan/coala,nemaniarjun/coala,MariosPanag/coala,Balaji2198/coala,stevemontana1980/coala,jayvdb/coala,refeed/coala,MattAllmendinger/coala,FeodorFitsner/coala,jayvdb/coala,d6e/coala,arush0311/coala,mr-karan/coala,refeed/coala,nemaniarjun/coala,scriptnull/coala,SambitAcharya/coala,jayvdb/coala,coala-analyzer/coala,coala-analyzer/coala,Asalle/coala,ManjiriBirajdar/coala,arafsheikh/coala,refeed/coala,ManjiriBirajdar/coala,arjunsinghy96/coala,incorrectusername/coala,karansingh1559/coala,yland/coala,CruiseDevice/coala,netman92/coala,svsn2117/coala,AbdealiJK/coala,vinc456/coala,stevemontana1980/coala,sagark123/coala,sudheesh001/coala,ayushin78/coala,sils1297/coala,Balaji2198/coala,tltuan/coala,SanketDG/coala,nemaniarjun/coala,MattAllmendinger/coala,Uran198/coala,NalinG/coala,yashtrivedi96/coala,JohnS-01/coala,coala/coala,AdeshAtole/coala,Tanmay28/coala,AdeshAtole/coala,Tanmay28/coala,Uran198/coala,JohnS-01/coala,scriptnull/coala,scriptnull/coala,svsn2117/coala,RJ722/coala,Tanmay28/coala,MariosPanag/coala,sophiavanvalkenburg/coala,swatilodha/coala,NalinG/coala,abhiroyg/coala,NiklasMM/coala,shreyans800755/coala,ayushin78/coala,RJ722/coala,tltuan/coala,arjunsinghy96/coala,karansingh1559/coala,SambitAcharya/coala,Tanmay28/coala,Nosferatul/coala,Shade5/coala,impmihai/coala,meetmangukiya/coala,damngamerz/coala,dagdaggo/coala,kartikeys98/coala,Tanmay28/coala,FeodorFitsner/coala,arush0311/coala,yland/coala,incorrectusername/coala,yashtrivedi96/coala,vinc456/coala,sophiavanvalkenburg/coala,rresol/coala,impmihai/coala,sudheesh001/coala,rimacone/testing2,scriptnull/coala,AbdealiJK/coala,djkonro/coala,andreimacavei/coala,meetmangukiya/coala,yashLadha/coala,CruiseDevice/coala,dagdaggo/coala,yashtrivedi96/coala,NalinG/coala,arush0311/coala,yashLadha/coala,MattAllmendinger/coala,Tanmay28/coala,stevemontana1980/coala,coala/coala,d6e/coala,AdeshAtole/coala,damngamerz/coala,abhiroyg/coala,svsn2117/coala,kartikeys98/coala,NalinG/coala,sils1297/coala,SambitAcharya/coala,NalinG/coala,dagdaggo/coala,SambitAcharya/coala,tushar-rishav/coala,scriptnull/coala,tushar-rishav/coala,vinc456/coala,andreimacavei/coala,saurabhiiit/coala,scottbelden/coala,scriptnull/coala,lonewolf07/coala,sagark123/coala,shreyans800755/coala,aptrishu/coala,rresol/coala,ayushin78/coala,NalinG/coala,netman92/coala,scottbelden/coala,netman92/coala,kartikeys98/coala,Shade5/coala,Nosferatul/coala,NalinG/coala,Balaji2198/coala,rresol/coala,Asnelchristian/coala,impmihai/coala,SanketDG/coala,Shade5/coala,saurabhiiit/coala,NiklasMM/coala,ManjiriBirajdar/coala,arafsheikh/coala,Asnelchristian/coala,coala/coala,Tanmay28/coala,scottbelden/coala,rimacone/testing2,Asalle/coala,incorrectusername/coala,CruiseDevice/coala,abhiroyg/coala,saurabhiiit/coala,JohnS-01/coala,sils1297/coala,djkonro/coala,FeodorFitsner/coala,SanketDG/coala,SambitAcharya/coala,sudheesh001/coala,shreyans800755/coala,sagark123/coala,karansingh1559/coala,MariosPanag/coala,rimacone/testing2,SambitAcharya/coala,djkonro/coala,Asnelchristian/coala,tushar-rishav/coala,andreimacavei/coala,AbdealiJK/coala,lonewolf07/coala,yland/coala,NiklasMM/coala,d6e/coala,tltuan/coala,Nosferatul/coala,Uran198/coala,yashLadha/coala,Asalle/coala,lonewolf07/coala,mr-karan/coala,sophiavanvalkenburg/coala,aptrishu/coala,coala-analyzer/coala,scriptnull/coala