Themes: Add additional later escaping to `post_class()` and `body_class()` functions.

Additionally, this adds a few tests to test output.

Fixes #20009.

Props mfields, scribu, azaozz, obenland, dd32, nacin, jrf, jdgrimes, garyj, whyisjake. 
 

Built from https://develop.svn.wordpress.org/trunk@48060


git-svn-id: http://core.svn.wordpress.org/trunk@47827 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/post-template.php

@@ -454,7 +454,7 @@ function has_excerpt( $post = 0 ) {
  */
 function post_class( $class = '', $post_id = null ) {
 	// Separates classes with a single space, collates classes for post DIV.
-	echo 'class="' . join( ' ', get_post_class( $class, $post_id ) ) . '"';
+	echo 'class="' . esc_attr( join( ' ', get_post_class( $class, $post_id ) ) ) . '"';
 }
 
 /**
@@ -590,7 +590,7 @@ function get_post_class( $class = '', $post_id = null ) {
  */
 function body_class( $class = '' ) {
 	// Separates class names with a single space, collates class names for body element.
-	echo 'class="' . join( ' ', get_body_class( $class ) ) . '"';
+	echo 'class="' . esc_attr( join( ' ', get_body_class( $class ) ) ) . '"';
 }
 
 /**

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5-alpha-48059';
+$wp_version = '5.5-alpha-48060';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.