add unittest for core_collapse

related to #41

I read this issue and thought I could take a stab (we had already several tests ported for polymer-elements).

First I tried the test for `<core-ajax-dart>` but I wasn't able to get anything but `<debug target crashed>`.
This test is checked in this test to my cloned repo.
 
The test for `<core-collapse>` works fine so far.

If this is what you want, I can add/port the other tests too.
no need to change it, but FYI - we've for some reason avoided the enhanced config and used just html_config. I think it doesn't matter in this case, but the html_config has nice support for running individual tests among other things.

I hope that one day we don't have to pick a config in the test, but let the test runner pick that for you :)