Internal: Add tests

[kaicataldo]

Now that the feature set is stabilizing a bit, the plugin desperately needs tests. Any help is appreciated!

[dpfavand]

Do you have any preferences for testing library?

Jest + testing-library/react seems like a pretty common combination that probably has a relatively low learning curve for contributors.

[kaicataldo]

Yeah, that does seem pretty common in the React world! Makes sense to test the withHydration HOC and the hydrated bundle. What I was initially thinking was:

1. Unit testing withHydration
2. Integration tests where we run the Eleventy build (maybe a nested full project with a package.json and everything?) and then assert that the generated files have the expected content. We could then load up the site in a headless browser and make assertions about the hydrated component behavior.

Thoughts?