Submitted to WPT as https://github.com/web-platform-tests/wpt/pull/52598
but in the meantime here's a local version.
The spec for this isn't super thorough, so the tests are based on how
Chrome and Firefox behave. Specifically, Firefox returns the ltr/rtl
keyword in lowercase but Chrome keeps the original case for it.
We currently fail most of these but that will be fixed in subsequent
commits.
