check: remove require_{test,scratch}* after a test fails

check: remove require_{test,scratch}* after a test fails

Remove the require_{test,scratch]* sentinel files after a test fails.
This eliminates false fsck corruption reports such as the following:

1. Test A calls _require_scratch, which creates the sentinel file
$RESULT_DIR/require_scratch to facilitate fsck after the test completes.

2. Test A runs some test, which corrupts the scratch filesystem due to
kernel bug or something.

3. Test A calls _fail because of the errors in (2).  Note that the test
case returned 1, so ./check unmounts the test and scratch filesystems
without checking them or removing $RESULT_DIR/require_scratch

4. Test B starts up, but does not call _require_scratch.  The
$RESULT_DIR/require_scratch file is still there.

5. Test B completes successfully.

6. ./check calls _check_filesystems, which sees the
$RESULT_DIR/require_scratch file and runs fsck.

7. fsck reports the corrupt scratch device (which is associated with
test B) even though B did not ever touch the scratch device and it was
actually test A that corrupted the filesystem.

Note that with the "check: wipe scratch devices between tests" patch
applied, we can also reproduce this problem by running xfs/172 and
xfs/195 with a scratch device small enough that the files created in 172
span multiple AGs and therefore cause 172 to fail.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>