LU-18354 tests: avoid sanity/136 OOM on ZFS servers

LU-18354 tests: avoid sanity/136 OOM on ZFS servers

Creating 150k files in sanity test_136 is causing OOM issues on
ZFS servers.  I'm not sure what might be causing that, but it is
disruptive to all patch review testing.  Rather than stop testing
on ZFS it is better to flush memory periodically on the MDS so the
test can continue to test the intended changelog functionality.

Test-Parameters: trivial
Test-Parameters: testlist=sanity env=ONLY=136,SLOW=yes,ONLY_MINUTES=60 fstype=zfs
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I4856d80a911eebcfcdaff2d2a1c91ef49645f0b4
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57036
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>