LU-14139 statahead: add test for batch statahead processing

LU-14139 statahead: add test for batch statahead processing

This patch adds sanity test_123ad() for batched statahead
processing, verify it work as expected.

Test-Parameters: trivial testlist=sanity env=ONLY=123ad,ONLY_REPEAT=50
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I742dcac7140ad842fff2afadeb2d948661365b95
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/41220
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mikhail Pershin <mpershin@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>