MB-8244: Unit test fix in fetching-worker-stats

Change-Id: I0831bf56410f76b0c9849fffab0f8fcbfaff4b6e
Reviewed-on: http://review.couchbase.org/29856
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>