minor #33349 [Form] fix parameter type declaration and make fabbot happy (xabbuh)

This PR was merged into the 3.4 branch.

Discussion
----------

[Form] fix parameter type declaration and make fabbot happy

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Commits
-------

19b292893a fix parameter type declaration and make fabbot happy

src/Symfony/Component/Form/FormInterface.php

@@ -31,7 +31,7 @@ interface FormInterface extends \ArrayAccess, \Traversable, \Countable
      * @throws Exception\LogicException            when trying to set a parent for a form with
      *                                             an empty name
      */
-    public function setParent(FormInterface $parent = null);
+    public function setParent(self $parent = null);
 
     /**
      * Returns the parent form.

src/Symfony/Component/Form/Tests/FormFactoryTest.php

@@ -13,8 +13,9 @@ namespace Symfony\Component\Form\Tests;
 
 use PHPUnit\Framework\MockObject\MockObject;
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\EventDispatcher\EventDispatcher;
+use Symfony\Component\Form\FormBuilder;
 use Symfony\Component\Form\FormFactory;
-use Symfony\Component\Form\FormInterface;
 use Symfony\Component\Form\FormTypeGuesserChain;
 use Symfony\Component\Form\Guess\Guess;
 use Symfony\Component\Form\Guess\TypeGuess;
@@ -194,7 +195,7 @@ class FormFactoryTest extends TestCase
             ->method('buildForm')
             ->with($this->builder, $resolvedOptions);
 
-        $form = $this->createMock(FormInterface::class);
+        $form = $this->createForm();
 
         $this->builder->expects($this->once())
             ->method('getForm')
@@ -227,7 +228,7 @@ class FormFactoryTest extends TestCase
             ->method('buildForm')
             ->with($this->builder, $resolvedOptions);
 
-        $form = $this->createMock(FormInterface::class);
+        $form = $this->createForm();
 
         $this->builder->expects($this->once())
             ->method('getForm')
@@ -467,6 +468,13 @@ class FormFactoryTest extends TestCase
         $this->assertSame($this->builder, $this->builder);
     }
 
+    protected function createForm()
+    {
+        $formBuilder = new FormBuilder('', null, new EventDispatcher(), $this->factory);
+
+        return $formBuilder->getForm();
+    }
+
     private function getMockFactory(array $methods = [])
     {
         return $this->getMockBuilder('Symfony\Component\Form\FormFactory')