Migrate existing tests to pytest from unittest.

[mrhappyasthma]

Existing tests are not running as part of the github actions. I believe it's because I implemented them using unittest and we are now using pytest (which seems to be the preferred options from some googling).

[kocielnik]

Good note!

I actually should've included the line python run_all_tests.py in the new GitHub Actions config, because it was there in the CircleCI config.

I don't think migrating to PyTest is required to get these tests to work.

While it's not required, I do think it will unify the test logic.

Before we migrate to PyTest, we make sure some tests (not only "API" ones) run at all for incoming PRs.

That was my goal in #81.

[kocielnik]

My, that was quick.

Thank you! :)