private String replyWrongDistributionToCreateVisitor(MockSender sender,
                                                         String clusterState) {
        CreateVisitorMessage msg = (CreateVisitorMessage)sender.getAndRemoveMessage(0);
        WrongDistributionReply reply = new WrongDistributionReply(clusterState);
        reply.setMessage(msg);
        reply.addError(
                new com.yahoo.messagebus.Error(
                        DocumentProtocol.ERROR_WRONG_DISTRIBUTION,
                        "i pity the fool who uses 1 distribution bit!"));
        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