fix race in FakeSubscriberServiceImpl

This is the same race as
#1495
but in sendStreamResponse rather than sendError.

Author: pongad

google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/spi/v1/FakeSubscriberServiceImpl.java

@@ -208,6 +208,7 @@ public void sendStreamingResponse(StreamingPullResponse pullResponse)
       throws InterruptedException {
     waitForRegistedSubscription();
     synchronized (openedStreams) {
+      waitForOpenedStreams(1);
       openedStreams.get(getAndAdvanceCurrentStream()).responseObserver.onNext(pullResponse);
     }
   }