Add coverage for media#player, move body class to view (#31790)

app/views/media/player.html.haml

@@ -2,6 +2,8 @@
   = render_initial_state
   = javascript_pack_tag 'public', crossorigin: 'anonymous'
 
+- content_for :body_classes, 'player'
+
 :ruby
   meta = @media_attachment.file.meta || {}