Replace karma because it is deprecated

[nolanlawson]

Karma was recently deprecated.

For running browser-based Jasmine tests, the likely replacement seems to be Web Test Runner and web-test-runner-jasmine.

[nolanlawson]

Related: #3083

[git2gus]

This issue has been linked to a new work item: W-13710350

[nolanlawson]

Whatever we use as a replacement, ideally it should:

1. Run Jasmine, as our existing Karma tests do
2. Run in browsers, and support SauceLabs
3. Have a good debug story
4. Have a good test coverage story, as we currently compile test coverage in Circle CI
5. Be stable, so we're not in this same position in another few years

If we can't find a good replacement, we should either 1) stick with Karma for now, or 2) write our own simple test runner.