[Symfony] Persist doctrine.dbal.backend_connection if Doctrine2 module is used

Fixes #3479

Author: Naktibalda

src/Codeception/Module/Symfony.php

@@ -202,6 +202,9 @@ public function _getEntityManager()
             if ($this->_getContainer()->has('doctrine.orm.default_entity_manager')) {
                 $this->persistService('doctrine.orm.default_entity_manager', true);
             }
+            if ($this->_getContainer()->has('doctrine.dbal.backend_connection')) {
+                $this->persistService('doctrine.dbal.backend_connection', true);
+            }
         }
         return $this->permanentServices[$this->config['em_service']];
     }