These apply to any elements that have some kind of open/closed state.
The spec suggests `<details>`, `<dialog>`, and `<select>`, so that's
what I've supported here. Only `<details>` is fleshed out right now,
but once the others are, these pseudo-classes should work
automatically. :^)
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.
