Add test suite

[svenluijten]

This PR:

• Adds a basic integration test suite to prevent regressions.
• Sets up GitHub Actions to run the tests automatically on every PR and push to the main branch.
• Adds unit tests for relevant classes.

Closes #3.

[svenluijten]

Let me know if you want (me) to add more unit tests @simonhamp! Most of the unit-testable code would just be testing setters/getters, and that feels pretty wasteful 😅

[simonhamp]

This looks excellent. I just had one question about the GitHub Action workflow

[svenluijten]

I should note that I haven't been able to test this workflow. So it might require some tweaking after it's been merged into main.

[simonhamp]

Let's get it merged and see what happens!

[simonhamp]

@svenluijten it looks like the GitHub Actions are struggling to write the image files produced by the tests. Could be due to permissions?

https://github.com/simonhamp/the-og/actions/runs/7435530543/job/20230963612