Unit and end-to-end tests

[simon360]

This library should be thoroughly tested.

The unit tests from wel-ui covered most of the two packages, but had some holes. Since this is basically a rewrite, the tests will also need rewriting. However, the old tests did cover some useful edge cases.

[willtonkin]

As we'd discussed @simon360, you'd made a start on this. Do you want to continue to own soley or could we break this down into smaller tasks then I can help?

[simon360]

Happy for any help!

Rather than overthink it, might be worth doing PRs file-by-file. Otherwise, we could spend a lot of time writing issues for each category of test that we want.

I already started in on react-from-markup/src/rehydrator.ts, and I'll take another look at what I've done tomorrow. May be that what I did is worthless - was a few months ago,