Execute W3C web platform tests as part of test suite

See #666.

This is my rather simple draft for now which should work for a majority of the web platform tests. I looked over the default test reporter and it didn't seem to handle things much differently.

I'm not too sure how to handle the complete w3c test suite, we have a **lot** of failing tests which need to be fixed, so I'd hold off of just importing the whole test suite and manually put the urls in there for now, which makes the process already much easier in comparsion to before. Not too sure what to do about the manual entry of urls; any comments?

Also any nitpicks for merging?
Also, this variable name is bad. And `body` isn't great either, as in `request` context it means "response body," but in DOM contexts it means `<body>`.