minor #21686 Updated PHPUnit namespaces (peterrehm)

This PR was merged into the 3.2 branch.

Discussion
----------

Updated PHPUnit namespaces

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Follow Up of #21663

Commits
-------

c2e80e3 Updated PHPUnit namespaces

Author: nicolas-grekas

Tests/ChoiceList/Loader/CallbackChoiceLoaderTest.php

@@ -11,13 +11,14 @@
 
 namespace Symfony\Component\Form\Tests\ChoiceList\Loader;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Form\ChoiceList\LazyChoiceList;
 use Symfony\Component\Form\ChoiceList\Loader\CallbackChoiceLoader;
 
 /**
  * @author Jules Pietri <jules@heahprod.com>
  */
-class CallbackChoiceLoaderTest extends \PHPUnit_Framework_TestCase
+class CallbackChoiceLoaderTest extends TestCase
 {
     /**
      * @var \Symfony\Component\Form\ChoiceList\Loader\CallbackChoiceLoader

Tests/Extension/Core/DataTransformer/DateIntervalTestCase.php

@@ -11,7 +11,9 @@
 
 namespace Symfony\Component\Form\Tests\Extension\Core\DataTransformer;
 
-abstract class DateIntervalTestCase extends \PHPUnit_Framework_TestCase
+use PHPUnit\Framework\TestCase;
+
+abstract class DateIntervalTestCase extends TestCase
 {
     public static function assertDateIntervalEquals(\DateInterval $expected, \DateInterval $actual)
     {