Add smoke test adapted from ROS 1 smoke test

[jtbandes]

Re-enable the smoke test from develop on the ROS 2 branch.

• Uses launch_testing instead of a .test xml file.
• The server now allows choosing port:=0 to select an ephemeral port, and sets the port number in the actual_port ROS param (similar to the Autobahn server implementation on develop). The test uses the GetParameters service to retrieve the port number.

I used various tricks to get async/await working in the test. This was difficult because rclpy has its own executor that doesn't interoperate with other event loops (like twisted or asyncio). Since the test uses both ROS publishers/subscribers and a Twisted WebSocket client, both event loops are needed, so I ran rclpy.spin_until_future_complete in a thread managed by Twisted. Mostly this worked nicely, but required care to use reactor.callFromThread in certain places, and required some explicit executor.wake() since the rclpy executor didn't automatically wake up when futures were resolved.