Change testing framework to Jest

The boilerplate I used uses web-runner, which is nice because

• it loads up a browser and tests stuff as they would be used by the user
• it's relatively fast because files do not need to be compiled to bundled Javascript to be loaded (using es imports instead).

Downsides:

• Need Chrome browser (also in the CI pipeline, which is tedious in Docker containers)
• We don't benefit from it when we don't run it locally.
• The browser needs to start, and with a small project, that may take more time than the compilation process.

With Jest (used by the react team) the test will run headless by emulating components.

We can always go back but at this point Jest is the pragmatic approach because we can just install and use it in docker containers without installing a frame buffer, window manager and Chrome in it.