KSES: Support the `video` element's `playsinline` attribute.

Allow users without the `unfiltered_html` capability to use the `playsinline` attribute when embedding videos.

Additionally this adds unit tests for passing the video element through kses.

Fixes #50167. See #29826.

Built from https://develop.svn.wordpress.org/trunk@47837


git-svn-id: https://core.svn.wordpress.org/trunk@47613 1a063a9b-81f0-0310-95a4-ce76da25c4cd