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

• [x] assignments (see #40)
• [ ] courses
• [ ] docs
• [ ] errors
• [ ] submissions
• [ ] venvs

Frontend Apps Remaining

Additionally, it's important to make sure buttons and stuff still appear where they should. These are the apps that need to be covered by frontend tests.

• [ ] assignments
• [ ] courses
• [ ] docs
• [ ] 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>.