Each ref test now links to its reference page with a link tag, in the
same format as WPT:
`<link rel="match" href="reference-page.html" />`
The reference pages have all been moved into a separate `reference/` dir
so that we can just treat every file in `ref/` as a test. There's no
filter to only look at .html files, because we also have a .svg file in
there, and there may be other formats we want to use too. But it's not
too hard to add one if we need it.
The ref tests runner takes screenshots of both the input page and the
expected page, then compares them. Ref testing allows us to catch
painting bugs, which cannot be detected with the layout and text tests
we already have.
With ref tests, we'll likely want to reuse the same expectation page
for multiple inputs. Therefore, there's a `manifest.json` file that
describes the relationship between inputs and expected outputs.
