Adding tests to Tin

[JasonGrace2282]

After #28 got merged, it's now possible to add backend tests to Tin.

Backend Apps Remaining

These are the apps that need to be covered by backend logic tests

• assignments (see Add tests for the assignments app #40)
• courses
• errors
• submissions (see Add tests for Submissions app #79)
• venvs

Frontend Apps Remaining

• Set up a frontend testing harness with Playwright

After setting up a test harness, the following apps should be covered. Covering means ensuring that the necessary buttons appear/do not appear on the correct pages.

• assignments
• courses
• submissions
• venvs

[JasonGrace2282]

A framework for testing UI has not yet been satisfactorily implemented. The last approach involved parsing html elements into a flattened list, which was just not efficient. An HTML testing framework should ideally allow us to just has has_button().with_text().with_url() instead of looking for a <a href="..."></a>.