Tests: Remove the ::append_to_selector() method from `Tests_Theme_w…

…pThemeJson`.

The method exists in the `WP_Theme_JSON` class and appears to be erroneously duplicated in the test class.

Add a unit test that was meant to be included instead, as per WordPress/gutenberg#32190.

Follow-up to [51051].

See #52991.
Built from https://develop.svn.wordpress.org/trunk@51054


git-svn-id: http://core.svn.wordpress.org/trunk@50663 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-51053';
+$wp_version = '5.8-alpha-51054';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.