It's great to see that you're embracing testing. Duality is an amazingly stable piece of code for it's size, but as it grows and hopefully becomes more of a community driven effort, tests will make a huge difference in the quality.
I just wanted to bring up one of your guidelines for when to write tests. You said
When writing a new game component or engine feature you’re still experimenting with: Don’t Unit-Test that.
I just wanted to mention Test Driven Development and see if that's something you're familiar with. TDD actually helps to improve the design of your APIs because the tests become basically like any other client of your code, and help to show you where your code is too coupled by being difficult to write. In TDD, you write a small test first, then write the code to make the test pass. If you're looking for a good example of TDD tests, check out scriptcs. They're very nicely written in that (and also it's a project we're investigating to add scripting support to Duality. Early tests are promising:) ).
That said, I'm all for hacking some code together to explore an idea first, so long as that code is thrown away and re-written with tests.