Build/Test Tools: Fix issue with add method in object-cache.php.

In the object-cache.php file used for unit tests, the add method did not work as expected. Other object cache plugins and core, have a check to see if the key exists in memory before writing it. Without this check, it used to write unnecessarily to the cache.  

Props spacedmonkey, SergeyBiryukov.
Fixes #57963.
Built from https://develop.svn.wordpress.org/trunk@55577


git-svn-id: http://core.svn.wordpress.org/trunk@55089 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.3-alpha-55574';
+$wp_version = '6.3-alpha-55577';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.