Refactor unit testing docs section

[natekupp]

This section needs work. There's a statement of "So let’s do that. Follow along in the comments:" that doesn't actually lead to any comments. Also, this section isn't written similarly to real-world unit test code. I feel like it would be better to demonstrate this with something like @pytest.mark.parametrize to show a more useful test case.

[schrockn]

I'm working on testing so I can do this

[mgasner]

These docs have been rewritten since this issue was opened.