It's not realistic to enable full-blown type checks
within test_runner's codebase, since the amount of
warnings revealed by mypy is overwhelming.
Tests are supposed to be easy to use, so we can't
cripple everybody's workflow for the sake of imaginary benefit.
Ultimately, the purpose of this attempt is three-fold:
* Facilitate code navigation when paired with python-language-server.
* Make method signatures apparent to a fellow programmer.
* Occasionally catch some obvious type errors.
