[#7941] use class constant

Author: xabbuh

form/unit_testing.rst

@@ -174,6 +174,7 @@ allows you to return a list of extensions to register::
 
     use AppBundle\Form\Type\TestedType;
     use Symfony\Component\Form\Extension\Validator\ValidatorExtension;
+    use Symfony\Component\Form\Form;
     use Symfony\Component\Form\Forms;
     use Symfony\Component\Form\FormBuilder;
     use Symfony\Component\Form\Test\TypeTestCase;
@@ -193,7 +194,7 @@ allows you to return a list of extensions to register::
                 ->will($this->returnValue(new ConstraintViolationList()));
             $validator
                 ->method('getMetadataFor')
-                ->will($this->returnValue(new ClassMetadata('Symfony\Component\Form\Form')));
+                ->will($this->returnValue(new ClassMetadata(Form::class)));
 
             return array(
                 new ValidatorExtension($validator),