Add a warning if a file yields no test classes.
Add a warning if a file yields no test classes.

Currently we get an opaque error when dereferencing None.

cevaris/commons,imsut/commons,cevaris/commons,foursquare/commons-old,ericzundel/commons,Yasumoto/commons,pombredanne/commons,atollena/commons,imsut/commons,jsirois/commons,nkhuyu/commons,abel-von/commons,abel-von/commons,atollena/commons,nkhuyu/commons,brutkin/commons,WCCCEDU/twitter-commons,WCCCEDU/twitter-commons,WCCCEDU/twitter-commons,pombredanne/commons,VaybhavSharma/commons,ericzundel/commons,WCCCEDU/twitter-commons,cevaris/commons,VaybhavSharma/commons,pombredanne/commons,nkhuyu/commons,cevaris/commons,ericzundel/commons,pombredanne/commons,brutkin/commons,VaybhavSharma/commons,VaybhavSharma/commons,VaybhavSharma/commons,imsut/commons,abel-von/commons,atollena/commons,Yasumoto/commons,nkhuyu/commons,jsirois/commons,foursquare/commons-old,atollena/commons,abel-von/commons,jsirois/commons,foursquare/commons-old,brutkin/commons,Yasumoto/commons,brutkin/commons,Yasumoto/commons,abel-von/commons,Yasumoto/commons,WCCCEDU/twitter-commons,cevaris/commons,jsirois/commons,atollena/commons,nkhuyu/commons,brutkin/commons,jsirois/commons,ericzundel/commons,ericzundel/commons,pombredanne/commons