Review of widgets tests

Hi Kai,

I have started to review the work you've done on testing widgets: great
work! It looks like a very good infrastructure to develop tests for the
Widgets specs.

I'm afraid they look sufficiently good that I don't have any specific
suggestions for improvement at this stage :) I can imagine creating more
tests cases, but that should probably wait till we hear back from the
WebApps WG - I'll send the Chairs a message right away (my bad that I
didn't do that earlier).

Dom