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Fixes #10661

Also update _TESTS

Ref: #10625

In a strict sense, <track>s with kind=captions are not subtitles. [1]
openload misuses this attribute, and I guess there will be more
examples, so I add it to common.py.

Also allow extracting information for subtitles-only <video> or <audio>
tags, which is the case of openload.

[1] https://www.w3.org/TR/html5/embedded-content-0.html#attr-track-kind