Support VoD assets with wvtt subtitles

[tobbee]

wvtt subtitles should be supported if available in a test VoD asset. The right signalling must be present in the asset. The media timestamps should be rewritten similarly to what is done for video and audio.

Note, there doesn't seem to be any test content for livesim1, so this use case may not have been run before. Further note that the automatic generation of timed subtitles is a way of testing wvtt subtitles.

Also add test content to the repo, so that there is something to test.