RFC: Use __getitem__ and __setitem__ instead of stuff/sub_stuff about autotest-docker

[cevich]

I don't like hiding data or behaviour when it's not necessary to support the fundamentals of a class. In this case, a "Test" is is not a container of things, it's a executor of actions. Using stuff/sub_stuff makes test code very clear, easy to debug. It also provides an aspect that's common to nearly every test and that anyone can easily understand. I'm open to changing the names if you can come up with something better, but I'm quite attached to this simple and unambiguous construct.