[WIP] run mypy on tests/ directory

[danieleades]

mypy is currently not run on the tests directory.

This PR addresses that.

There's a massive number of errors- too many to introduce in one PR. Hence I'm whitelisting the existing issues to be incrementally improved.

[picnixz]

Actually, I didn't want to be run on tests/ for now. I just wanted to have the --follow-imports=normal so let's do this first I think. Also, there are many mistakes that I found when modernizing the test suite (because we don't use mypy there, PyCharm was helpful and I could see that some tests were not correctly written sometimes!)