add a test for media:playback-active-class

[marisademeglio]

Media Overlays in EPUB 3.0.1 introduces a new metadata property called "media:playback-active-class"

[danielweck]

Note that this feature is supported in Readium (including the publicly-available Chrome extension).

[marisademeglio]

There is already a test for this (mo-playback-010). Closing the issue with no changes.