As I understand it, Eve uses a core of C++ for the rendering, and Python for 'once or less per frame' stuff, like AI. So Python might comprise most of the codebase by linecount, but it's not in the intensive innermost loop. Of course, my needs are very modest, so I'm confident that I can go the whole hog in Python. But I think that will go a lot farther than people generally believe. And one day, when the JIT lands in core Python, suddenly it'll be a whole new ballgame.
Yep, I TDD all my personal projects if I think I'm going to be changing or maintaining it for a while. But if I'm in a game jam with a team of people, you can't insist on that - folks aren't into it. And things that are throwaway I'm less strict about.
As for resources, well, it's distinctly outside of the 'games' field, but IMHO the single best book on testing is "Test Driven Development with Python", aka "The Goat Book" http://www.obeythetestinggoat.com/. It shows how to create a Python web project using the Django framework, but in a TDD style. Disclaimer, it was written by a former colleague and friend of mine, and I was a technical reviewer. But although I'm biased, I'm really seriously impressed by what he achieved. He really dived into the topic, producing both more depth and more breadth than I expected, seriously tackling thorny real-world problems head on. He produced something that I honestly think is a classic software book, and it's very lively and readable, without dumbing down.