add test for core-media-query

I tried to do the test `core-media-query` within the main browser window but I learned that it is not supported to resize the current window (except once in onLoad) and I wasn't able to find another way to make `core-media-query` fire a change event, events so I went with the dialog window.
The test runs fine so far, didn't even need a `new Future.delayed` for synchronization.
I had planned to write more tests and reuse the MyModel class, but now I don't see much gain in additional tests, therefore it makes perfectly sense to move these definitions to MyModel.