FIX: Sensitivity of `cvmfs_umount()` in test_functions

The helper function `cvmfs_umount()` was too sensitive when cross-checking a successful unmount in `/proc/mounts`. This lead to failing tests when `cvmfs_umount atlas.cern.ch` was called while there was a file system mounted on `/srv/cvmfs/atlas.cern.ch/cache` for example. Fixed.