See https://github.com/w3c/web-platform-tests/tree/master/webvtt
As I commented on bug 203783, we've literally never run anything from ./media/track/opera/track/webvtt/rendering/reftest/, which I guess means we have less test coverage than we thought. It's unlikely to be worthwhile spending any time getting that directory running and passing, versus getting the modern tests in WPT running.
I'm running into an issue importing these as they timeout in run-webkit-tests, but not in the browser. If anyone knows how to resolve that'd be appreciated.
The tests in webvtt/parsing (https://github.com/web-platform-tests/wpt/tree/master/webvtt/parsing) has been imported by https://bugs.webkit.org/show_bug.cgi?id=261671