Add test for `picture` element support, per the proposed spec/impl. currently underway.

I understand if you guys want to hold off on merging this one until we see some real-world implementations, but wanted to sneak this over to you in advance—we made sure to add `window.HTMLPictureElement` to the spec for the sake of easy testing. Happy to own any feature testing issues that should pop up when this thing starts landing in stable browsers, as well.