LU-12865 tests: fix sanity 160f to be more robust

LU-12865 tests: fix sanity 160f to be more robust

The sanity test_160f test was failing intermittently because the first
Changelog user ("cl6") was being unregistered in some cases when it
set changelog_max_idle_time=10, but the test slept for 9s and then did
some operations that could be slow.  In rare cases the test runs too
long and the MDS evicts the "good" user along with the bad user:

   MDD0000: Force deregister of ChangeLog user cl7 idle more than 35s
   MDD0000: Force deregister of ChangeLog user cl6 idle more than 11s

Change the test sleep interval to be half of the max_idle limit so
that there is no risk of the "good" Changelog user being evicted.

Add some logging to the test so that it is easier to correlate test
script actions with events in the MDS debug log.

Fixes: 31fef6845e8b ("LU-10680 mdd: create gc thread when no current transaction")
Test-Parameters: trivial envdefinitions=ONLY=160 testlist=sanity,sanity
Test-Parameters: envdefinitions=ONLY=160 mdscount=2 testlist=sanity,sanity
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I0e4c9c271d98a2716f848e75676780b0383ebbe5
Reviewed-on: https://review.whamcloud.com/36468
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>