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.
These two ref-tests involve two boxes positioned in the same place, with
outlines. Outlines always have a border-radius, meaning that the corner
pixels are not 100% opaque. (It seems to be 254 instead of 255.) With
the test files painting two outlines, and the ref test only painting
one, slight changes in the background color of the page would make that
slight variation visible sometimes. So, let's avoid that inconsistency
by always painting one outline instead of two.
