LU-10970 tests: make sure write is complete

LU-10970 tests: make sure write is complete

The current test does not guarantee the write has arrived
on the server before dropping caches and checking memory
usage.  If the write is still in progress, the baseline
memory used value will be incorrect.

Sync on the client to force the write out.

Test-Parameters: trivial

Cray-bug-id: LUS-5923
Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: Ic0379ffdfd14ff630d65a0197a99fba929868e9c
Reviewed-on: https://review.whamcloud.com/32203
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>