CBQE-0 :: Fix test case due to race condition for persistence

Change-Id: I6589fe70a457b4f3053a4227cd42e00d2b904245
Reviewed-on: http://review.couchbase.org/41255
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Parag Agarwal <agarwal.parag@gmail.com>