Improve the testing of WebPlatform

[vicb]

Currently WebPlatform spec uses whatever is available on the browser that executes the tests (shadow DOM or shadow DOM polyfill).

This means that you should executed the tests on two different platforms to get a significant result. That's why I think this is more like an e2e test than a unit test.

It would be good to add some real unit tests that do not rely on the execution in a browser to get more significant result.

As the WebPlatform class as an hardcoded dependency on js.context['Platform'], one way to do that would be inject there a mock platform before the tests (and restore the original value afterwards).

This could help testing #1304.