VisitorParameters createVisitorParameters(String selection) {
        VisitorParameters params = new VisitorParameters(selection);
        params.setRoute("storage"); // cannot be null by default
        // TODO: skip the above and rather mock cluster route resolution, since
        // this must be supported anyway!
        return params;
    }
Mock the executor to keep things nicely single threaded for the testing.
No need to synchronize things here since we don't use multiple threads.
documentapi/src/test/java/com/yahoo/documentapi/messagebus/MessageBusVisitorSessionTestCase.java