Include element HTML in QUnit summary for diff

Because trying to work out how broken your test case is
by looking at two (potentially multi-page) serialisations
of the DOM summary is a pain in the arse. Diffing two
HTML strings may highlight the problem much more clearly.

TODO: Is it possible to defer the calculation of the HTML
infused summary object until we have determined the assertion
has failed. Otherwise we're slowing down our tests for no
reason.

Change-Id: I873bf2479ab81d15389792bd59d15580da63941a
Merge "Include element HTML in QUnit summary for diff"