xfs: wrap xfs_db calls to the test device

Create a _test_xfs_db analogue to _scratch_xfs_db so that we can
encapsulate whatever strange test fs options were fed to us by the test
runner.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>