@Override
        public void onMessage(Message m, AckToken token) {
            builder.append("onMessage\n");
            messages.add(new MessageWrapper(m, token));
            if (exceptionOnMessage != null) {
                throw exceptionOnMessage;
            }
        }
Test that using an id.user=foo selection only tries to visit a single
superbucket for that user.
documentapi/src/test/java/com/yahoo/documentapi/messagebus/MessageBusVisitorSessionTestCase.java