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.
This has to cheat and use a screenshot but thanks to the "Take Full
Screenshot" feature of Ladybird, it is very easy to update this test.
The steps are documented in the test.