Remove reference to the test container after kernel shutdown

This reference is not working anymore, as the main container was reset and
cleaned. Keeping a reference to the test container will prevent collecting
the object graph.

src/Symfony/Bundle/FrameworkBundle/Test/KernelTestCase.php

@@ -123,6 +123,7 @@ abstract class KernelTestCase extends TestCase
                 $container->reset();
             }
         }
+        static::$container = null;
     }
 
     /**