HTML API: Ensure form closer tokens are reachable.

Form tag closers have complicated conditions. There was a bug where the HTML Processor would not stop correctly on a FORM tag closer token.
This changeset ensures that that token is reachable.

Developed in https://github.com/WordPress/wordpress-develop/pull/7582
Discussed in https://core.trac.wordpress.org/ticket/61576

Follow-up to [58779].

Props jonsurrell.
Fixes #61576.
Built from https://develop.svn.wordpress.org/trunk@59248


git-svn-id: http://core.svn.wordpress.org/trunk@58640 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/html-api/class-wp-html-processor.php

@@ -2433,6 +2433,7 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 					}
 
 					$this->state->stack_of_open_elements->remove_node( $node );
+					return true;
 				} else {
 					/*
 					 * > If the stack of open elements does not have a form element in scope,