Add tests for plugin commandline options

[ryanking13]

This adds tests for plugin cmdline options --dist-dir, --runner, --runtime, while removing a test-config that makes #59 fail.

[hoodmane]

I find that these don't work as expected:

pytest --rt chrome some/file.py # won't work 
pytest --rt=chrome some/file.py # works
pytest some/file.py --rt chrome # works 

[ryanking13]

I find that these don't work as expected:

Yeah.. accepting a list-type parameter in argparse is quite a bit annoying. Maybe we should change it to get a comma-separated list of runtimes instead? e.g. --rt "chrome,firefox,node"