deflake SubscriberImplTest (#1495)

This commit tries to deflake failures reported in https://ci.appveyor.com/project/garrettjonesgoogle/google-cloud-java/build/57 .

The failure seems to come from `FakeSubscriberServiceImpl::sendError`. When calling `getAndAdvanceCurrentStream`, we waited for there to be a subscription by waiting on the `subscriptionInitialized` variable.

The variable is notified before `addOpenedStream` is called though, and so it's possible that `getAndAdvanceCurrentStream` will see a list without any opened stream. The fix is to wait for at least one stream to be registered first.

google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/FakeSubscriberServiceImpl.java

@@ -264,6 +264,7 @@ public void modifyAckDeadline(
   public void sendError(Throwable error) throws InterruptedException {
     waitForRegistedSubscription();
     synchronized (openedStreams) {
+      waitForOpenedStreams(1);
       Stream stream = openedStreams.get(getAndAdvanceCurrentStream());
       stream.responseObserver.onError(error);
       closeStream(stream);