[tests] migrate from jest to vitest (#3940)

This is a big change, but I think it's a good move, as `vitest` is much
more modern than `jest`.

I'm excited about the UI watch feature (run `npm run test:ui`), for
example - it's really helpful and saves time when debugging tests. I had
to adjust a few tests because they had time related issues, but
basically we are now testing the same things - even a bit better and
less flaky (I hope).

What do you think?

.gitignore

@@ -88,3 +88,6 @@ js/positions.js
 # Ignore lock files other than package-lock.json
 pnpm-lock.yaml
 yarn.lock
+
+# Vitest temporary test files
+tests/**/.tmp/