Migrate from WCT (web-component-tester) to Karma

[Hexcles]

Despite being advertised in the official Polymer doc, WCT seems to be falling out of favour nowadays. It has many issues (see our past gripes and its public issue tracker) and not actively maintained anymore.

We also have a ticking time bomb here: https://github.com/web-platform-tests/wpt.fyi/blob/ce53fbb315af1ab3b14e442203c3f7fcc32590f9/webapp/wct.conf.json#L12

A quick survey both on the internet and internally within Google suggests that Karma seems to be the preferred solution.