minor #7941 added missing ClassMetadata (OskarStark)

This PR was merged into the 2.7 branch.

Discussion
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added missing ClassMetadata

This needs to be merged into the following branches too

Closes #7270
Refs symfony/symfony#20952

Commits
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c193035 added missing ClassMetadata

Author: xabbuh

form/unit_testing.rst

@@ -178,6 +178,7 @@ allows you to return a list of extensions to register::
     use Symfony\Component\Form\FormBuilder;
     use Symfony\Component\Form\Test\TypeTestCase;
     use Symfony\Component\Validator\ConstraintViolationList;
+    use Symfony\Component\Validator\Mapping\ClassMetadata;
     use Symfony\Component\Validator\Validator\ValidatorInterface;
 
     class TestedTypeTest extends TypeTestCase
@@ -190,6 +191,9 @@ allows you to return a list of extensions to register::
             $validator
                 ->method('validate')
                 ->will($this->returnValue(new ConstraintViolationList()));
+            $validator
+                ->method('getMetadataFor')
+                ->will($this->returnValue(new ClassMetadata('Symfony\Component\Form\Form')));
 
             return array(
                 new ValidatorExtension($validator),