ApcuStorage: user cache from APC

I've copied the tests from memcached and rewritten them for the new storage. The problem is that Apcu clears cache depending on TTL only [when new request occurs](http://php.net/manual/en/function.apc-store.php) - which means It cannot be easily tested.

Any idea how to test it?

Or should I implement "fallback" expiration check that will store when the key should be expired and then expire it manually if neccesary?