LU-17675 tests: flush opencache in sanity-flr/61a

LU-17675 tests: flush opencache in sanity-flr/61a

flush opencache to update MDS's atime with close RPC

Lustre-change: https://review.whamcloud.com/54788
Lustre-commit: 3e37a49ec072a65cfa76bbb242e125450bdc9676

Test-Parameters: trivial testlist=sanity-flr clientdistro=el9.3
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I5f4d3400b3f772553ee6004ac271a4aa644699e0
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55461
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>