Reduce code duplication in tests

- Define `train_dir`, `test_dir`, etc. in `tests` (i.e. in its `__init__.py`) and then import those variables into all test scripts so that they don't need to constantly be redefined in each `tests` module.
- Shape up test tear down/setup methods: There's a lot of repetitive code. And, with `pathlib`, for example, one can use `Path.unlink(missing_ok=True)` to eliminate a lot of logic.