@Override
        public void onDone(CompletionCode code, String message) {
            super.onDone(code, message);
            builder.append("onDone : ").append(code).append( " - ");
            builder.append("'").append(message).append("'\n");
        }
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