Tests: Correctly back up and restore theme directories in `Tests_Theme`.

In the `set_up()` method, a copy would be made of the original value of the global `$wp_theme_directories` variable, with the intention to restore that original value in the `tear_down()` method after running each test. Unfortunately, this was not implemented correctly.

* The backup is made to a function local variable in `set_up()` and not stored somewhere where it is accessible from the `tear_down()` method.
* The restoring then references a non-existent property to restore, which would effectively set the `$wp_theme_directories` global to `null`.

Fixed by declaring and using a private property to store the original `$wp_theme_directories` value.

This bug was discovered while fixing (removing) the magic methods in the `WP_UnitTestCase_Base` class in an effort to improve compatibility with PHP 8.2.

Follow-up to [38907].

Props jrf, costdev, johnbillion, swissspidy.
See #55652.
Built from https://develop.svn.wordpress.org/trunk@54040


git-svn-id: http://core.svn.wordpress.org/trunk@53599 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-54039';
+$wp_version = '6.1-alpha-54040';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.