Refactor Test Suite #25

Most relevant comment:

I think the way we want to refactor this might be to something similar to python/mypy@master/test-data/unit

That looks horrible haha. I'd prefer a more IDE-friendly approach like https://github.com/numpy/numtype/tree/main/src/numpy-stubs/%40test , possibly with automatic test-generation like numtype also does.

In https://github.com/scipy/scipy-stubs/tree/master/tests I explained the mechanisms of this testing method a bit better.

Originally posted by @jorenham in #18 (comment)