fix test case for platforms without truncate utility
fix test case for platforms without truncate utility

djw8605/cvmfs,djw8605/cvmfs,Gangbiao/cvmfs,djw8605/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,Gangbiao/cvmfs,djw8605/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs,djw8605/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs