LU-15977 test: improve sanityn 80b

LU-15977 test: improve sanityn 80b

Backport sanityn test_80b change from LU-15529.

Lustre-commit: 1d6b96a1cf0468bc81949960aa649cde8f927008
Lustre-change: https://review.whamcloud.com/40891

Test-Parameters: trivial mdtcount=4 mdscount=2 testlist=sanityn env=ONLY=80b,ONLY_REPEAT=20
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ie3fb71ddc7de9df32baa45851c21522cc939d6bd
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55478
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>