Karma is deprecated

[CendioOssman]

Describe the bug
Our test runner, Karma, has been deprecated for over a year. It is now starting to break, with Chai moving to modules and none of the bindings between Karma and Chai seem to be maintained to handle this transition.

To Reproduce
Steps to reproduce the behavior:

1. Run npm test
2. Tests fail to run, complaining about module import/export

Expected behavior
Tests run just fine.

Client (please complete the following information):
N/A

Server (please complete the following information):
N/A

Additional context
https://github.com/karma-runner/karma#karma-is-deprecated-and-is-not-accepting-new-features-or-general-bug-fixes

[CendioOssman]

I manage to get things up an running on web-test-runner, but tests hang on failure because of this bug:

modernweb-dev/web#2772

[CendioOssman]

Band-aid in 1b2fe33. Unknown how long until the next thing breaks.