Wait for the graph to settle in DroppingTheStreamUnsubscribes

[yukiendo-pfr]

TopicStreamTest.DroppingTheStreamUnsubscribes (added in #61) checks count_subscribers() right after creating and after dropping the subscription. The graph is updated asynchronously, so the count can still be stale at that point, and the test fails intermittently under load:

test_topic_stream.cpp:223: Failure
Expected equality of these values:
  node_->count_subscribers("dropped_stream_topic")
    Which is: 0
  1u

The test now polls count_subscribers() until it reaches the expected value (up to 5 s) before asserting.

Running 16 instances in parallel (each in its own ROS_DOMAIN_ID) for 320 runs in total: 57 failures before, 0 after.

[otamachan]

LGTM Thanks