fate/subtitles: Add a new test for WebVTT

Includes escapes that should now be supported and a few features not yet
fully supported, like comments, regions, classes, ruby, and lang.

All were tested with https://quuz.org/webvtt/

 for validation, except
regions because the validator doesn't support them yet, and I couldn't
find any other way to validate WebVTT.

Signed-off-by: Ricardo Constantino <wiiaboo@gmail.com>