CBQE-985: add flag test to cas tests

Change-Id: Ia1162b72c61bc05ee333cd3aa3c20c9b0a278aee
Reviewed-on: http://review.couchbase.org/24318
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>