Improve tests

[stevenvachon]

• added separate tests for .add()
• added tests for adding/removing multiple classes
• all HTML tests now have SVG variants for better coverage
• clarified test titles for .toggle() as to differentiate them from the other methods
• switched to current latest 2.x version of QUnit

[beck]

Not yet willing to give up on upstream: https://github.com/eligrey/classList.js/issues/63

[beck]

I've rewritten all the tests and implemented e2e scaffolding and a bit of CI so the polyfill gets tested across browsers.

Closing this in favor of: https://github.com/eligrey/classList.js/pull/69