Fix sporadic test failure in unit-tests

Change-Id: I2322f0975f7c1ff7f0bbf21b50c844a4abd7ee81
Reviewed-on: http://review.couchbase.org/37090
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>