LU-18321 tests: skip ost-pools/31 for older MDS

LU-18321 tests: skip ost-pools/31 for older MDS

Skip the loop spilling testing in ost-pools test 31 for
MDS < 2.15.50.150 since the test depends on changes made
to the MDS code on the server.

Test-Parameters: trivial testlist=ost-pools env=ONLY=31 \
  serverversion=2.15.5

Change-Id: I4ad2ded8073eca55363fe60cc49c580ca7d983ae
Fixes: c9c842d678 ("LU-15043 lod: check for spilling loops")
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56657
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sarah Liu <sarah@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>