@Override
        public VisitorResponse getNext(int timeoutMilliseconds) throws InterruptedException {
            builder.append("getNext : ").append(timeoutMilliseconds).append('\n');
            return new VisitorResponse(null);
        }
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