Clarify effects on test task

[velovix]

Right now, the test task defined in this extension is a thin wrapper around the Python test task defined by colcon-core, but with PyTest forced on. However, looking at the code, there's no reason why the oddly named setup.py testing mode wouldn't work. It just appears to run tests using unittest.

The only thing that will definitely not work is colcon-core's automatic testing mode detection. It uses dependencies specified in the user's setup.py to figure out whether to use PyTest or unittest automatically. It looks like the code is written with fallbacks in case that information is not available, but it's worth investigating if we can do some auto-detection of our own.