generic/013: encapsulate remount during cleanup

generic/013: encapsulate remount during cleanup

The existing code calls remount for $TEST_DEV with constantly defined
mount options. This can fail if a user specifies different mount options.
Fix this by using new _test_remount() call that remounts $TEST_DEV.

Signed-off-by: Pavel Shilovsky <pshilovsky@samba.org>
Reviewed-by: Steve French <smfrench@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>