Replace the JS test runner

[stof]

Instead of the outdated setup using the unmaintained phantomjs, this now uses @web/test-runner and runs tests in an actual browser.

I also updated QUnit to its latest version instead of the totally outdated version used before. To run tests locally, I kept the index.html that supports standalone run of QUnit, even though npm test -- --manual is also possible thanks to the manual mode of @web/test-runner