Pytest fixture to wrangle different implementations behind a facade

[vytas7]

Brought up by @webknjaz in PR comment:

In terms of setting up fixtures, @webknjaz wants to share something he came up with over in aio-libs:

• https://github.com/aio-libs/multidict/blob/fe4dee0/tests/conftest.py#L24-L156
• https://github.com/aio-libs/propcache/blob/bcb84bf/tests/conftest.py#L12-L64.

This strategy is partially inspired by a technique of wrapping params as dataclasses, that one of the pytest maintainers was showing at a EuroPython tutorial + some of @webknjaz's derivative ideas.