LU-14361 statahead: Add test for statahead advise

LU-14361 statahead: Add test for statahead advise

This patch adds a test program "aheadmany" and sanity/test_123g to
verfiy that statahead advise works as expected.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I751313ecb790c66f70a09bf8e9d13846b3c0032d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51730
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>