make S3 Uploader test fixture tear down more robust
make S3 Uploader test fixture tear down more robust

cvmfs/cvmfs,djw8605/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,trshaffer/cvmfs,alhowaidi/cvmfsNDN,djw8605/cvmfs,alhowaidi/cvmfsNDN,DrDaveD/cvmfs,alhowaidi/cvmfsNDN,alhowaidi/cvmfsNDN,DrDaveD/cvmfs,cvmfs/cvmfs,djw8605/cvmfs,cvmfs/cvmfs,alhowaidi/cvmfsNDN,djw8605/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,trshaffer/cvmfs,cvmfs/cvmfs,trshaffer/cvmfs,cvmfs/cvmfs,trshaffer/cvmfs,djw8605/cvmfs,DrDaveD/cvmfs,trshaffer/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,Gangbiao/cvmfs,Gangbiao/cvmfs