We should provide some buildless testing in Data flows

We set up test first very clearly in the previous module, and then throw it over completely when we enter the DOM.

It would be reasonably simple to provide a miniature tester that runs in the browser and provides a few of the same assertions. We used Jest-lite in our old ITD, but we could equally use

https://alexwlchan.net/2023/testing-javascript-without-a-framework/
https://lit.dev/playground/#gist=05655736300f0425644e61a626410aef

etc