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.
git-svn-id: https://develop.svn.wordpress.org/trunk@55577 602fd350-edb4-49c9-b593-d223f7449a82