replace Karma with @web/test-runner

The `karma` test runner is [deprecated and no longer maintained](https://github.com/karma-runner/karma#karma-is-deprecated-and-is-not-accepting-new-features-or-general-bug-fixes).

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