minor #21663 Updated PHPUnit namespaces (peterrehm)

This PR was squashed before being merged into the 2.8 branch (closes #21663).

Discussion
----------

Updated PHPUnit namespaces

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Follow Up of #21564

Commits
-------

205ced4 Updated PHPUnit namespaces

Author: nicolas-grekas

Tests/Extension/DependencyInjection/DependencyInjectionExtensionTest.php

@@ -11,10 +11,11 @@
 
 namespace Symfony\Component\Form\Tests\Extension\DependencyInjection;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException;
 use Symfony\Component\Form\Extension\DependencyInjection\DependencyInjectionExtension;
 
-class DependencyInjectionExtensionTest extends \PHPUnit_Framework_TestCase
+class DependencyInjectionExtensionTest extends TestCase
 {
     public function testGetTypeExtensions()
     {

Tests/Util/StringUtilTest.php

@@ -11,9 +11,10 @@
 
 namespace Symfony\Component\Form\Tests\Util;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Form\Util\StringUtil;
 
-class StringUtilTest extends \PHPUnit_Framework_TestCase
+class StringUtilTest extends TestCase
 {
     public function testTrim()
     {