LU-10576 tests: allow log files to be created/removed

LU-10576 tests: allow log files to be created/removed

Allow an llog file to be created or removed during the course of
the test, as this can happen due to internal housekeeping activity.

Also ensure that background cleanup has finished with ZFS before
fetching the number of objects from the MDT.

Test-Parameters: trivial
Test-Parameters: testlist=sanity mdscount=2 mdtcount=4 mdtfilesystemtype=zfs ostfilesystemtype=zfs
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I0a0968cfcd90c7493c67b54ba8a7f326163ebbe5
Reviewed-on: https://review.whamcloud.com/33677
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>