LU-16307 tests: fix sanity-sec test_31

LU-16307 tests: fix sanity-sec test_31

In order to improve sanity-sec test_31 resiliency, reorganize the way
the new LNet '999' is handled. And make sure everything is correctly
cleaned up after the test.

Lustre-change: https://review.whamcloud.com/53818
Lustre-commit: TBD (from f4a96799159fd662855542d471197ac4060d3295)

Test-Parameters: trivial testgroup=review-dne-part-2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Idd657c7555e598d0ebc08387eac537b1c73e35bd
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53779
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>