private String replyToCreateVisitor(MockSender sender, BucketId progress) {
        CreateVisitorMessage msg = (CreateVisitorMessage)sender.getAndRemoveMessage(0);
        CreateVisitorReply reply = createReply(msg);
        reply.setLastBucket(progress);
        sender.reply(reply);
        return createVisitorToString(msg);
    }
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