deflake SubscriberImplTest, maybe

Author: pongad

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);