LU-16589 tests: add sanity/31l to test ln command

LU-16589 tests: add sanity/31l to test ln command

This patch adds a new subtest sanity/31l to test
hard-linking a file to a target direcory that has
trailing "/".

The subtest will be skipped if the coreutils version
>= 8.31 and kernel version < 5.18 because the
coreutils commit v8.30-18-g571f63f5010b reveals
a kernel issue, which is fixed by kernel commit
v5.18-rc2-188-gb3d4650d82c7.

Test-Parameters: trivial clientdistro=el7.9 env=ONLY=31l testlist=sanity
Test-Parameters: trivial clientdistro=el8.7 env=ONLY=31l testlist=sanity
Test-Parameters: trivial clientdistro=el9.1 env=ONLY=31l testlist=sanity
Test-Parameters: trivial clientdistro=el9.0 env=ONLY=31l testlist=sanity
Test-Parameters: trivial clientdistro=sles15sp4 env=ONLY=31l testlist=sanity
Test-Parameters: trivial clientdistro=sles15sp3 env=ONLY=31l testlist=sanity

Change-Id: I45d7c277a37fa538d5137150bfc7ba1704052873
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50265
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>