Disable tests that use document.characterSet

The hubbub bindings set this value but we don't use it for anything.  html5ever
doesn't parse charsets from <meta> tags yet, and I'd rather add that as part of
adding actual support for other encodings (which Servo never had with hubbub).
Skip WPT tests for <img srcset>

Switching to html5ever changes attribute parsing slightly (bringing it in line
with the current spec) and this breaks some of the <img srcset> tests.  We
don't support this feature at all; there's no reason we should have been
passing before.
Use html5ever for HTML parsing