common: don't check scratch dev on all tests

Some tests deliberately corrupt  scratch devices and so will fail
the post-test check. Add a "_require_scratch_nocheck" helper
function for such tests to avoid false test failure detection.

Also, ensure that _notrun cleans up the trigger for the post-test
checks. Otherwise the next test to run may try to validate the
scratch/test devices even though they are not used by the test.

Further, _check_xfs_filesystem() causes check to exit if it finds a
corruption. This is extremely annoying as it terminates the entire
test run rather than just reporting that the test fails. Hence add
an "iam != check" test before exiting so that calls from tests will
cause the test to fail, but calls from check won't cause the harness
to exit.

There are still some tests that fail the scratch check, these are
not obvious test failures and so need further investigation to
determine the cause of the failures.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/xfs/002

@@ -50,8 +50,8 @@ _cleanup()
 # Modify as appropriate.
 _supported_fs xfs
 _supported_os Linux
+_require_scratch_nocheck
 
-_require_scratch
 # So we can explicitly turn it _off_:
 _require_xfs_mkfs_crc