xfs/341: update test for rtgroup-based rmap

xfs/341: update test for rtgroup-based rmap

Now that we're sharding the realtime volume into multiple allocation
groups, update this test to reflect the new reality.  The realtime rmap
btree record and key sizes have shrunk, and we can't guarantee that a
quick file write actually hits the same rt group as the one we fuzzed,
so eliminate the file write test since we're really only curious if
xfs_repair will fix the problem.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>