Tests: Add unit tests for WP_Object_Cache::stats

[Infinite-Null]

Summary

Fixes a fatal error in WP_Object_Cache::stats() when the cache contains non-serializable objects like SimpleXMLElement.

Trac ticket: https://core.trac.wordpress.org/ticket/21650
Patch: https://core.trac.wordpress.org/attachment/ticket/21650/21650.diff

Testing

npm run test:php -- tests/phpunit/tests/cache.php

Screenshot:

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