LU-14330 tests: wait for orphan thread to exit

LU-14330 tests: wait for orphan thread to exit

It may take a few seconds for the orphan cleanup thread to finish.
Wait for the thread to exit rather than failing the test.

Test-Parameters: trivial
Fixes: a1e6e75a82 ("LU-12846 tests: verify orphan upgrade compatibilty")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I604be1b9f8f460d9183ba1aaddd3b77e153ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56559
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Alex Deiter <adeiter@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>