MB-8606: Unit test fix with multi threaded warmup

Change-Id: I6e86ae1cc8dbee62a9045c937dea931f1f7d89d4
Reviewed-on: http://review.couchbase.org/29556
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>