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.
Rather than modify the transform of the parent (which could change
independently), this adds a new override element_transform() where
element specific tranfroms can be applied. This will always stay in
sync with the attributes.
A ref test comparing a .svg and .html version of the same file is
added as due to differences in attribute parsing order, the .svg version
was previously drawn incorrectly.
Fixes#20859
