bug #25940 [Form] keep the context when validating forms (xabbuh)

This PR was merged into the 2.7 branch.

Discussion
----------

[Form] keep the context when validating forms

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #25542
| License       | MIT
| Doc PR        |

Commits
-------

317da3bdf8 keep the context when validating forms

src/Symfony/Component/Form/Extension/Validator/Constraints/FormValidator.php

@@ -124,6 +124,7 @@ class FormValidator extends ConstraintValidator
                     : gettype($form->getViewData());
 
                 if ($this->context instanceof ExecutionContextInterface) {
+                    $this->context->setConstraint($constraint);
                     $this->context->buildViolation($config->getOption('invalid_message'))
                         ->setParameters(array_replace(array('{{ value }}' => $clientDataAsString), $config->getOption('invalid_message_parameters')))
                         ->setInvalidValue($form->getViewData())
@@ -144,6 +145,7 @@ class FormValidator extends ConstraintValidator
         // Mark the form with an error if it contains extra fields
         if (!$config->getOption('allow_extra_fields') && count($form->getExtraData()) > 0) {
             if ($this->context instanceof ExecutionContextInterface) {
+                $this->context->setConstraint($constraint);
                 $this->context->buildViolation($config->getOption('extra_fields_message'))
                     ->setParameter('{{ extra_fields }}', implode('", "', array_keys($form->getExtraData())))
                     ->setInvalidValue($form->getExtraData())

src/Symfony/Component/Form/Tests/Extension/Validator/Constraints/FormValidatorTest.php

@@ -51,6 +51,8 @@ class FormValidatorTest extends AbstractConstraintValidatorTest
         $this->serverParams = $this->getMockBuilder('Symfony\Component\Form\Extension\Validator\Util\ServerParams')->setMethods(array('getNormalizedIniPostMaxSize', 'getContentLength'))->getMock();
 
         parent::setUp();
+
+        $this->constraint = new Form();
     }
 
     protected function getApiVersion()