Tests: Correct the `WP_Test_Stream::mkdir()` method.

The method attempted to check if there is already a file with the same name, however the conditional used an undefined variable.

This commit prevents directory creation if a file or directory with the same name already exists, bringing consistency with the PHP `mkdir()` implementation.

Includes adding missing documentation for the method.

Reference: [https://www.php.net/manual/en/streamwrapper.mkdir.php PHP Manual: streamWrapper::mkdir()].

Follow-up to [49230].

Props david.binda, sadizaman, rajinsharwar, SergeyBiryukov.
Fixes #59406.
Built from https://develop.svn.wordpress.org/trunk@56998


git-svn-id: http://core.svn.wordpress.org/trunk@56509 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.5-alpha-56996';
+$wp_version = '6.5-alpha-56998';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.