Thank you for the explanation.
Using importlib or to implement the code to test inside a Python module is indeed possible.
However, I still prefer the _launch.py suffix, since this would conform to pep naming and enable unit tests without creating additional Python modules.
I would also be interested to hear from users that moving from ROS1 to ROS2 if the _launch.py syntax feels unnatural compared to .launch.py or the other way around.