Add "when to stop testing?" page

[jmafoster1]

What?

Expand the "Should we aim for 100% test coverage" section into a "When should I stop testing?" section that introduces coverage.py, how to run it, what the output looks like, etc.

Why?

After finding a bug in some software I'm writing (that actually does have 100% test coverage), I thought maybe it'd be good to add a tip to the "Should we aim for 100% test coverage?" section of the training along the lines of "If you find a bug, add a test that catches that bug so you know when you've fixed it and to make sure you don't break anything again". Then I realised that the training doesn't really explain what "coverage" actually is or how to measure it.