Add a way to wait until a Subscriber connects to a Publisher #271

I'm working on writing automated tests for rosjava nodes. The node I'd like to test publishes a message in response to a trigger. In the test, I'd like to simulate the trigger and assert that the message is published.

My implementation is as follows:

1. Start a rosjava master.
2. Start the node-under-test.
3. In the test, create a Subscriber for the topic.
4. Wait for the node-under-test's Publisher to connect to the Subscriber.
5. Simulate the trigger.
6. Assert that the MessageListener's onNewMessage method is called within a given timeout.

However, I'm struggling with "Wait for the node-under-test's Publisher to connect to the Subscriber". My implementation looks like this:

    Subscriber subscriber = connectedNode.newSubscriber(topicName, messageType);
    subscriber.addMessageListener(messageListener);

    CountDownSubscriberListener subscriberListener = CountDownSubscriberListener.newDefault();
    subscriber.addSubscriberListener(subscriberListener);
    if (!subscriberListener.awaitNewPublisher(timeout, unit)) {
      throw new RuntimeException("timeout waiting for publisher to connect");
    }

But there is a race condition - if the Publisher connects before the test calls addSubscriberListener, the test cannot know that a Publisher has connected.

One way around this would be to add a Subscriber.hasPublishers() method, analogous to Publisher.hasSubscribers(). To avoid breaking compatibility with existing code (is this a concern?) we could add it to DefaultSubscriber, at the cost of a more fragile test.

@ernestmc @jubeira What do you think? Is there a better way to write this test?