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.
what `nr` does stands for? Is it an abbreviation for number? If so, maybe we can spell out the name a bit, some ideas: `messageNum / message_num`, or `messageId / message_id`