LU-17784 build: improve wiretest for flexible arrays

LU-17784 build: improve wiretest for flexible arrays

Flexible array checking can additionally probe that the size
of the array element is correct.

Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: Ib7de3d156a2e77dfaf2e9ab1df8fab524c073610
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54929
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>