[Symfony] Make grabService use the special test service container if available (#5706)

grabService seems to be somewhat broken for newer Symfony versions (> 4.1 ?) and with Symfony 4.1 a special container can be accessed via `test.service_container` for testing purposes which also provides access to private services:

https://symfony.com/doc/current/testing.html#accessing-the-container

Author: burned42

src/Codeception/Module/Symfony.php

@@ -265,7 +265,17 @@ public function _getEntityManager()
      */
     public function _getContainer()
     {
-        return $this->kernel->getContainer();
+        $container = $this->kernel->getContainer();
+
+        if (!($container instanceof ContainerInterface)) {
+            $this->fail('Could not get Symfony container');
+        }
+
+        if ($container->has('test.service_container')) {
+            return $container->get('test.service_container');
+        }
+
+        return $container;
     }
 
     /**