Add a test suite

[waldyrious]

This would give us more confidence (other than manually testing the site) when merging PRs. It would also enable automatic version bumps.

Since this is a GUI app rather than a software library or CLI interface, it's likely that we will need some sort of visual testing, e.g. using Playwright, WebdriverIO, BackstopJS, etc. Here's a blog post describing how Wikipedia implemented such a visual regression testing system.