LU-18682 tests: ost-pools/23b to call wait_delete_completed

LU-18682 tests: ost-pools/23b to call wait_delete_completed

otherwise the next test 24 may find few OSTs almost filled
and do not use them for new objects leading to a failure:
 ost-pools test_24: @@@@@@ FAIL: Stripe count 4 not on
 	/mnt/lustre/d24.ost-pools/dir1/f24.ost-pools0:3

Test-Parameters: trivial
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I0136dc352603b8f9ccd8926b3105d99ce0df63e2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57917
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandre Ioffe <aioffe@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>