Mock a the minimal subset of WebRTC for testing.

The intent of this PR is to introduce a basic mocking of the WebRTC API. Just enough to make [shortwave](inetCatapult/shortwave) work with Zombie.js integration tests.

I am going to be adding tests to this PR.
This should be the whole `coverage/` directory and not just the HTML report. There are several coverage files in this commit that should be removed.