Tests: Add Node.js / SSR tests.

### The Problem

We test our code in browsers via Karma, but we don't test in Node.js for SSR, which many of our users rely on. This means that for errors like our latest [Node.js + new ESM bug](https://github.com/FormidableLabs/victory/issues/1957) we didn't catch up because it only manifests in Node.js.

### Suggested Work

- [ ] Implement simple Node.js SSR tests in Node.js
- [ ] Pick a testing library that makes sense.
- [ ] Probably use `lib` CommonJS transpiled code (we could use `es` for a future Node + ESM test scenario, but for starters maybe just Node + good ol' CJS)
- [ ] Ideally _don't_ transpile our test code. That would mean using something like [htm](https://github.com/developit/htm) instead of JSX. Or Vanilla JS.