Code Modernization: Remove unused dynamic property in `Tests_WP_Custo…

…mize_*`.

The dynamically created `$undefined` property appears to be a copy/paste from the `Tests_WP_Customize_Setting` class, and is not actually used in other test classes:

* `Tests_WP_Customize_Manager`
* `Tests_WP_Customize_Panel`
* `Tests_WP_Customize_Section`

Follow-up to [31329], [32658].

Props jrf.
See #56033.
Built from https://develop.svn.wordpress.org/trunk@53851


git-svn-id: http://core.svn.wordpress.org/trunk@53410 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53850';
+$wp_version = '6.1-alpha-53851';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.