Consider making a regression_data fixture in conftest.py

[jvshields]

#413 sets up regression data, but needs to import the regression_data class in to conftest.py. Ruff wants to remove the import, so we added an assert statement to make ruff not remove it.

This maybe points to the fact that we should have a regression_data pytest fixture initialized in conftest.py, since conftest.py is largely the place to create such fixtures and this change seems in line with that idea.