check: Make the test harness unmount fs when finishing a test

check: Make the test harness unmount fs when finishing a test

Finishing xfs/132 left a shutdown scratch fs and the test harness
didn't unmount the fs(because we told it not to check the fs) so the
test harness called by subsequent xfs/133 tried to "test -d
$SCRATCH_MNT" and received the IO error from the dead fs.

i.e. Running xfs/132 and xfs/133 together got the following error:
------------------------------------------------------------
...
xfs/132 1s ... 1s
xfs/133 1s ... [failed, exit status 1] - output mismatch (see /var/lib/xfstests/results//xfs/133.out.bad)
...
QA output created by 133
-Format and mount
-Corrupt filesystem
-Remount, try to append
-Write did not succeed (ok).
+SCRATCH_DEV=/dev/sda11 is mounted but not on SCRATCH_MNT=common/config: - aborting
+Already mounted result:
+/dev/sda11 /mnt/xfstests/scratch
...
------------------------------------------------------------

Even if we don't check fs, the test harness is supposed to unmount
fs and return an initial state before running the next test.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>