LU-18354 tests: speed up sanity/136 on non-ZFS

LU-18354 tests: speed up sanity/136 on non-ZFS

The previous change to sanity test_136 to improve test reliability
on ZFS servers resulted in the test time increasing by about 8x
(from ~300s to ~2400s).  Only wait for deletion and drop caches on
ZFS MDS nodes, and not on ldiskfs where this is not needed.

Test-Parameters: trivial
Test-Parameters: testlist=sanity env=ONLY=136,SLOW=yes,ONLY_MINUTES=30 fstype=zfs
Test-Parameters: testlist=sanity env=ONLY=136,SLOW=yes,ONLY_MINUTES=30
Fixes: 627cc62369 ("LU-18354 tests: avoid sanity/136 OOM on ZFS servers")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ic5dc79f9b7e6c2df50a97d0447ef3aa9d3c73e1d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58199
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Timothy Day <timday@amazon.com>