fstests: _fail test by default when _scratch_mount fails

fstests: _fail test by default when _scratch_mount fails

Previously _scratch_mount didn't check the mount status and most
tests continue to run even if the mount failed (unless test checks
for the mount status explicitly). This would result in running tests
on the underlying filesystem (usually rootfs) and implicit test
failures, and such failures can be annoying and are usually hard to
debug.

Now _fail test by default if _scratch_mount failed and introduce
_try_scratch_mount for tests that need to check mount results
themselves.

Suggested-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>