apex_launchtest bind test args to setup functions

[pbaughman]

@hidmic This shuffles the logic that binds attributes and arguments to tests out of apex_runner.py and into loader.py At the same time, I'm also using the argument binding trick to bind the args to the test setUp, setUpClass, tearDown, and tearDownClass methods so they can access the processes under test too.

I think your idea to use arguments instead of attributes was really good and I'll probably end up deprecating the attribute method in favor of the test arguments method, but I need to find a good time to do it because I'll need to update all of our internal tests.

Just putting this here to avoid conflicts with your stuff. We can either merge this now if you need to build on top of it, or it will go in as part of a larger "apex_launchtest_ros" PR with some attempts at writing better ROS examples using this tool

[pbaughman]

@hidmic That's a good point. I expected the loader to pick up regular test functions but upon actually trying it, it does not appear to. I'll investigate a bit

[pbaughman]

Ah, yes - the python unittest loader we're extending only cares about unittest.TestCases