Fix gitattributes such that GitHub ignores the test files for its language detection.
Fix gitattributes such that GitHub ignores the test files for its language detection.

dbeyer/benchexec,sosy-lab/benchexec,IljaZakharov/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,martin-neuhaeusser/benchexec,martin-neuhaeusser/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,martin-neuhaeusser/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,martin-neuhaeusser/benchexec,sosy-lab/benchexec,dbeyer/benchexec,sosy-lab/benchexec,IljaZakharov/benchexec,IljaZakharov/benchexec,dbeyer/benchexec,IljaZakharov/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec