Create proper (automated) tests

Moodle Tools currently is not systematically tested. Therefore, we should introduce a test framework (e.g., pytest) and continuously write new tests as we add functionality. Also the tests should be run automatically for each pull request:

• Select and integrate a test framework
• Write tests for existing functionality
• Integrate a tool/automation that checks the code coverage of tests
• Create a GitLab CI automation that runs the tests for each PR