Add tests for notify guard condition in node

[jacquelinekay]

Connects to ros2/rclcpp#209

[jacquelinekay]

I find this test is currently flaky for Connext but no other platform. Looking into it.

[wjwwood]

I'm not sure it's worth changing this test, but for the future you might consider using a future here rather than the timer_triggered boolean. Something like this:

std::thread spin_thread(executor_spin_lambda);
std::promise<void> timer_triggered_promise;
std::shared_future<void> timer_triggered_future(timer_triggered_promise.get_future());
auto timer = node->create_wall_timer(
  1_ms,
  [&executor, &timer_triggered_promise](rclcpp::TimerBase & timer)
{
  timer_triggered_promise.set_value();
  timer.cancel();
});

EXPECT_EQ(std::future_status::ready, timer_triggered_future.wait_for(100_ms));
executor.cancel();
spin_thread.join();

The advantage of this approach is that if the timer fails to trigger the test doesn't just hang, but instead it fails an expected check and the test can continue to try other things. Currently the only way this test fails is if the test timeout kills the test.

[jacquelinekay]

The test was flaky on Connext; I added a busy_wait_for_subscriber and decided to put it in its own file to share it between multiple tests: 35720ea