[ticket/12657] Add a test file for base case of command cache:purge

Add test file for command cache:purge

https://tracker.phpbb.com/browse/PHPBB3-12657

PHPBB3-12657
Actually I was wrong. I still don't know how to do otherwise but the problem is here : 
https://github.com/phpbb/phpbb/pull/2545/files#diff-d661828db7a44deea734718e37a708afR40