Handle "constraints" option in form unit testing

In the current documentation, although a mocked `ValidatorInterface` is being passed to the `FormTypeValidatorExtension`, the actual `validate()` method in it is returning null. This causes any test against a form type that utilizes the extension's `constraints` option to fail, because of the following code in `Symfony\Component\Form\Extension\Validator\EventListener\ValidationListener`:

```php
            // Validate the form in group "Default"
            $violations = $this->validator->validate($form);

            foreach ($violations as $violation) {
                // Allow the "invalid" constraint to be put onto
                // non-synchronized forms
                $allowNonSynchronized = Form::ERR_INVALID === $violation->getCode();

                $this->violationMapper->mapViolation($violation, $form, $allowNonSynchronized);
            }
```

Note the `foreach` loop that is expecting an array; currently the mocked object returns null and any test fails.

Since the documentation uses the `ValidatorExtension` as a specific example, I think it would be nice for the example code to handle this case, preventing the user from having to dig deeper into the code to discover the problem.
it should actually return a ConstraintViolationList