Cross-registrable-domain origins for WPT.

Hello, testers!

There are a few platform features that take the registrable domain into
account (e.g. the `example.com` bit of `www.example.com`).
https://tools.ietf.org/html/draft-west-first-party-cookies-06 is one that
I'm working on implementing, and I'd hoped to start converting my unit
tests to WPT to help other vendors along.

I can't do that, however, as all WPT tests run on the same registrable
domain (`w3c-test.org`). It would be lovely if we could support a second (`
not-w3c-test.org`, for instance). I filed
https://github.com/w3c/web-platform-tests/issues/2669 with the request, and
jgraham suggested that I poke this list as there are some complexities
around `{{host}}`, etc. that he'd like to discuss.

Thanks!

-mike