Testing: Add TDD discussion and example

[bernhold]

Should be coordinated with addition of TDD to hands-on as well.

[bernhold]

https://neverworkintheory.org/2021/09/16/analyzing-the-effects-of-tdd-in-github.html reports on a study that finds TDD does no good in practice. This is the sixth paper they're reported on that finds no benefit to TDD. Should we reconsider this idea?

[bernhold]

Greg has done this in both presentations and hands-on for the SC21 event. Likely there will be improvements once we gain experience. But I think we can close this issue (existence) and create new ones to suggest future improvements.