av1_videos = {
        p
        for p in glob.glob("**/*.mp4", recursive=True)
        if is_av1_video(p)
    }

    assert av1_videos == set()

Building a set just to check if it's empty is a bit more complexity than necessary. A more direct way that also bails out early:

    assert not any(is_av1_video(p) for p in glob.glob("**/*.mp4", recursive=True))

Equivalently (de Morgan's law):

    assert all(not is_av1_video(p) for p in glob.glob("**/*.mp4", recursive=True))