integrate into `launch_testing` or `launch_ros_testing`

[wjwwood]

Normally to test a launch file using launch_testing you would include the launch file directly. But to use the --secure option provided by this package, you have to instead run ros2 launch from a new launch description, essentially nesting ros2 launch into a launch_testing run. This is sub optimal because you lose the ability to check the output and return codes of individual processes that are being run.