Add options perform "fast" testing

Testing the codebase takes approximately 40 minutes due to experiment and training tests. We could make some of those tests skippable by passing push options to the CI.

Documentation: https://docs.gitlab.com/ee/user/project/push_options.html

The core of the idea is to decorate slow tests with @pytest.mark.slow. Then, when pushing, if the developer deems so, they can use git push -o ci.variable="PYTEST_ADDOPTS='-m (not slow)'" to avoid running slow tests.