LU-17679 tests: enable sanity/851

LU-17679 tests: enable sanity/851

Print start message in monitor_lustrefs just before it is ready
to read events.
In test script, wait for the start message to confirm that
monitor_lustrefs process is scheduled and executed.
In test script, assign monitor_lustrefs process a higher priority.

Test-Parameters: trivial
Test-Parameters: testlist=sanity env="ONLY=851,ONLY_REPEAT=1000"
Signed-off-by: Lei Feng <flei@whamcloud.com>
Change-Id: Ibc5dd2450e7a8b795dba355118ea4a95f4cd52af
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58141
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>