Refactor codebase to use ES6+ features

[SebastianZ]

The code syntax the tests are written in is pretty much outdated and in some parts quite hard to read.

To improve that I suggest to use newer JavaScript language and DOM features like classes, let and const, modules, destructuring, new Array methods, etc.

Having said that, this will also break the site for browsers that don't support those features.
So the features should be chosen carefully and when possible be polyfilled if not supported.

Furthermore, backwards compatibility shouldn't be a problem anymore once the data needed for #57 is in place. When that's available there is no need to run the tests on old browsesr anymore.

Sebastian