LU-14869 test: improve sanity-flr/200a

LU-14869 test: improve sanity-flr/200a

Make sure "flock -x" successfully returned before running mirror
resync so that it won't get into running read holding shared flock.

Lustre-change: https://review.whamcloud.com/54345
Lustre-commit: TBD (from 2ff02fec932ab6e00a4942517db76f48412db31a)

Test-Parameters: trivial testlist=sanity-flr env=ONLY=200a,ONLY_REPEAT=10
Test-Parameters: trivial testlist=sanity-flr env=ONLY=200a,ONLY_REPEAT=10
Test-Parameters: trivial testlist=sanity-flr env=ONLY=200a,ONLY_REPEAT=10
Test-Parameters: trivial testlist=sanity-flr env=ONLY=200a,ONLY_REPEAT=10
Test-Parameters: trivial testlist=sanity-flr env=ONLY=200a,ONLY_REPEAT=10
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I6383af5d5761980d24af19efd4a4ac899f369a7d
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/54328
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>