Comments: Don't output "cancel comment reply link" if comments aren't threaded.

Though hidden via `style="display:none;"`, if the comments aren't threaded, this commit doesn't output the cancel comment reply link (skips over that logic). Change in `comment_form()`.

Adds tests.

Follow-up to [12810], [38959].

Props henrywright, jigneshnakrani, rachelbaker, desrosj, audrasjb, hellofromTonya.
Fixes #37267.
Built from https://develop.svn.wordpress.org/trunk@52175


git-svn-id: http://core.svn.wordpress.org/trunk@51767 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/comment-template.php

@@ -2538,11 +2538,13 @@ function comment_form( $args = array(), $post_id = null ) {
 
 		comment_form_title( $args['title_reply'], $args['title_reply_to'] );
 
+		if ( get_option( 'thread_comments' ) ) {
 			echo $args['cancel_reply_before'];
 
 			cancel_comment_reply_link( $args['cancel_reply_link'] );
 
 			echo $args['cancel_reply_after'];
+		}
 
 		echo $args['title_reply_after'];
 

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52174';
+$wp_version = '5.9-alpha-52175';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.