Adding getusermedia conformance tests.

So these are some tests we wrote a while back that I've updated and polished up so they follow the latest version of the spec. I'm not sure if we should rewrite them in the style of https://github.com/w3c/web-platform-tests/tree/master/mediacapture-streams, e.g. split them up a bit and perhaps enhance the existing tests instead.
I think most of these tests would be better done via idlharness.js https://github.com/w3c/testharness.js/blob/master/docs/idlharness.md (can't remember if I already have one of these in https://github.com/w3c/web-platform-tests/tree/master/mediacapture-streams )