cross_frame_test seems to refetch test URL on dartdevc, breaks test.py

[whesse]

We are getting errors on trybot that the test lib_2/html/cross_frame_test.dart is causing the framework to request 2 new tests when it finishes (perhaps finishing in two frames, or reloading), rather than 1 new test. The framework detects and reports this, and exits.

Skipping the test on DDC until we can figure out why it is doing this. None of the commands seems to actually reload the URL (It opens a new iframe with no URL, and never sets it). But it does expect not to be run in an iframe, so it should be failing anyway.