Add a test runner to muban 2 template so that we can write tests about pota

[ThaNarie]

@ThaNarie wrote a facade for testing-library that works with muban 2 iirc

Correct: https://github.com/mubanjs/muban-testing-library

There also is https://github.com/mubanjs/muban-test-utils, but that's more for testing hooks or muban internals.

In any case, the base pota skeleton might want to include Jest, so we can use that in any of our skeletons?

[psimk]

This is something that we would like to pick up with #42