Add testing library

[crookedneighbor]

Proposal to allow us to write tests for components. I chose to do just one test as a demo. If this looks good, we can add more , if not, we can just close this PR.

• Adds testing library for vue
• Adds tests for ThemeButton component

Considerations:

• I'm sure there's a better way to handle the vuex store, but I'm not sure what the strategy is
• I set the environment to jsdom, but we could use something like happy-dom if we wanted an alternative
• We could alternatively use vue test utils instead, but I really like testing library for it's focusing on encouraging best practices for accessibility with it's selectors

[scarletcs]

Perfect!! Getting something like this set up has been on my mental todo for a bit. :)

• Vuex store: Ideally, we run it integrated. We do not mock dispatch. We do not mock up its values. Instead, we have a fully instantiated store, we let the component dispatch updates to it, and we test that it updates as expected based on the state update it dispatched. My only concern there is finding the correct way to make Vuex and testing-libraries get along, given (unlike in the world of React) the store is not provided with a new one constructed per render() call.
• I've never actually used happy-dom, I'm good with jsdom.
• I'm a big fan of testing library too, so this is perfect to me.

[scarletcs]

I've asked them for some documentation on what we ought to do here:

• Documentation request: Vue testing library with Pinia testing-library/vue-testing-library#325
• Documentation request: Vue testing library with Vue Redux testing-library/vue-testing-library#326

If need be I'll even consider switching to Pinia. The app is tiny, our store is tiny, it won't be a huge deal to change, it'll be fun getting some exposure to Pinia.

[crookedneighbor]

I’ve heard really good things about Pinia!