LU-17660 tests: test symlink file to existing dir

LU-17660 tests: test symlink file to existing dir

Create a test case to verify LU-17660.

Symlink a file to an existing dir. Expect the symlink entry
under dir. Error if report "cannot overwrite directory".

Fixed in kernels newer than v6.9-rc4-39-gbb32cded3be2 or with
a backported fix.

Signed-off-by: Feng Lei <flei@whamcloud.com>
Test-Parameters: trivial
Test-Parameters: clientdistro=el8.10 testlist=sanity env=ONLY=17p
Test-Parameters: clientdistro=el9.4 testlist=sanity env=ONLY=17p
Test-Parameters: clientdistro=sles15sp5 testlist=sanity env=ONLY=17p
Test-Parameters: clientdistro=sles15sp6 testlist=sanity env=ONLY=17p
Test-Parameters: clientdistro=ubuntu2204 testlist=sanity env=ONLY=17p
Test-Parameters: clientdistro=ubuntu2404 testlist=sanity env=ONLY=17p
Change-Id: I905813c26e78ae3e6df4f88af10ab3f0c596a59b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56639
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>