CBQE-2131: new tests for graceful failover

Change-Id: I83ec336fc0e604e8b94a665742e81c14bcf93161
Reviewed-on: http://review.couchbase.org/34642
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>