Add launch tests

[christophfroehlich]

The old tests only test the URDF and if the view_robot.launch.py doesn't have a python syntax error and launch_ros does not complain.

The new tests for the main launch file now also check

• The python syntax is correct
• robot_state_publisher node is available
• the controller for this example is active
• joint_states topic is available and has the correct joints

closes #422

Deactivated tests or other temporary measures: I'll push draft PRs after merging this to not forget that!

• example_14 exit codes due to #542
• example_15 because of bug in spawners, may be solved with #502
• 0b8382b can be reverted once https://github.com/ros2/rclpy/issues/1320 is fixed

[christophfroehlich]

@saikishor I'll revert the changes to the cm update rate on top of #543, I fear that the high update rate will cause flaky tests without any log throttling.

[saikishor]

@saikishor I'll revert the changes to the cm update rate on top of #543, I fear that the high update rate will cause flaky tests without any log throttling.

Let's do that