add __init__.py to ensure package-level test discovery

[JDRanpariya]

No description provided.

[WT-MM]

I don't see the point of adding the test directory as a valid package? I think we just want to be able to run pytest.

Lmk your rationale for this

[JDRanpariya]

well for me when I wanted to run pytest -m "slow" --durations 10 I had to add PYTHONPATH=. as prefix, maybe I'm missing something, I guess at some point if tests get slow we might want to run it.

Maybe it's issue with my setup, I installed it with pip install .[dev] and not pip install -e .[dev], something we can add to contribute.md, then you're right we don't need it.