Migrate testing to pytest

[schmoelder]

Currently, we're still using standard library unittest for our test suite.
However, pytest offers a lot of advanced functionality for testing (e.g. improved fixtures) and is simpler to write.

Also, pytest can simply run standard library unit tests so migrating does not require rewriting all tests (although we should consider this in the medium term.

To do

• Call pytest in the CI/CD
• Add slow flags to tests that we don't always want to run (e.g. test_optimizer_behaviour)
• Skip tests when ax module is not found

See also #88, #144