Improve, simplify tests

[falquaddoomi]

As raised by @d33bs in #62 (comment) and #62 (comment), the tests have a few areas where they could be simplified or improved.

Some suggestions were:

• adding type hints to test function parameters
• removing commented-out lines, e.g. models in pytest.mark.parametrize calls, or at least explaining why they're commented out
• avoiding aliasing the tmp_path variable as output_folder in tests.

I imagine that we'd have a PR that just addresses these kinds of changes without changing any other functionality, to ease verification that the tests are still correct.