xfs/838: actually force usage of the realtime device

Actually force the filesystem to create testfile as a realtime file so
that the test does anything useful.

Cc: fstests@vger.kernel.org # v2025.07.13
Fixes: 7ca990e22e0d61 ("xfs: more multi-block atomic writes tests")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>