Update test expectations

The hubbub bindings set document.characterSet 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).

Switching to html5ever also 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.

Some other things change due to bugs in hubbub not present in html5ever.
Use html5ever for HTML parsing
Add a reftest for #1324

Closes #1324, fixed by html5ever.
auto merge of #3670 : kmcallister/servo/h5e-take2, r=jdm

r? @Ms2ger, @jdm

The parser is now a JS-managed object and we use hooks in html5ever to trace its internal state.  This should be memory-safe even if arbitrary JavaScript can run during a parse.  Please let me know if you think of a reason it wouldn't be!

I think the likely outcome of a garbage collection during parsing is a dynamic `RefCell` borrow failure, but I'm going to look into that after this lands.  It should be safe to trace the parser while it's mutably borrowed, as long as it's not shared between threads, so we can probably switch to `UnsafeCell`.