Code Modernization: Remove dynamic properties in WP_UnitTestCase_Base.

Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.

In this particular case, the `scandir()` method sets a dynamic `$matched_dirs` property, which is subsequently used in the `delete_folders()` method.

This commit removes the need for the property. Effectively, this changes the `scandir()` method to return an array of the matched directories instead of setting the property by using recursion in the method itself in an optimized manner.

Note the `array_merge()` not being in the loop itself, but at the very end of the function. This is for performance reasons, see [https://github.com/dseguy/clearPHP/blob/master/rules/no-array_merge-in-loop.md clearPHP: No array_merge() In Loops] for a more detailed explanation of this.

It has been verified in detail that the actual results of the previous version of the method and this version match, even when the paths passed are more complex and have deeper nested subdirectories.

Follow-up to [30658], [53557], [53558], [53850], [53851], [53852], [53853], [53854], [53856], [53916], [53935], [53936].

Props jrf, johnbillion, markjaquith, SergeyBiryukov.
See #56033.
Built from https://develop.svn.wordpress.org/trunk@53937


git-svn-id: http://core.svn.wordpress.org/trunk@53496 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53936';
+$wp_version = '6.1-alpha-53937';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.