replace Karma with @web/test-runner

[trusktr]

The karma test runner is deprecated and no longer maintained.

The new kid on the block is @web/test-runner, which is simpler to configure, faster, works with (vanilla!) ES Modules out of the box, has a much better live debug setup, and is just better in so many ways.

👇👌🤌 https://modern-web.dev/docs/test-runner/overview