Since this object cache implementation was added, WordPress has introduced a variety of caching API improvements:
* `wp_cache_add_multiple()`
* `wp_cache_set_multiple()`
* `wp_cache_get_multiple()`
* `wp_cache_delete_multiple()`
Although WordPress core provides a compatibility layer if these functions are missing from third-party object caches, this commit updates the Memcached object cache used in the test suite to implement these new functions directly.
Follow-up to [40561], [47938], [47944], [52700], [52703], [52706], [52708].
Props petitphp, spacedmonkey, tillkruss, SergeyBiryukov.
Fixes#54864.
git-svn-id: https://develop.svn.wordpress.org/trunk@54423 602fd350-edb4-49c9-b593-d223f7449a82