Is class A_B making use of the namespace? If so then you should be able to call it with is FQN EX:
use Acme\CoreBundle\Test\A_B;
Otherwise you need to use a require statement to load the file and that will load the class to the global namespace.
require_once 'Acme/CoreBundle/Test/A_B.php';
Im pretty sure thats how it works. Dont shoot me if im wrong.