In wp_list_comments(), update the comment meta cache when the comments derive from WP_Query and the new ->comment_meta_cached prop is false.

There are no uses of `wp_list_comments()` in Core where `$comments` are passed as the 2nd argument.

Adds unit tests.

Props wonderboymusic, bradt.
Fixes #16894.


git-svn-id: https://develop.svn.wordpress.org/trunk@33925 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/query.php

@@ -1292,6 +1292,15 @@ class WP_Query {
 	 */
 	 public $thumbnails_cached = false;
 
+	/**
+	 * Set if comment meta has already been cached
+	 *
+	 * @since 4.4.0
+	 * @access public
+	 * @var bool
+	 */
+	 public $comment_meta_cached = false;
+
 	/**
 	 * Cached list of search stopwords.
 	 *