fix occasionally failing test case.

Change-Id: I9c3d2c52e4232ba6355bb8ca68ebfa75fddbae5c
Reviewed-on: http://review.couchbase.org/40559
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>