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/Loader/ObjectRouteLoaderTest.php

@@ -11,11 +11,12 @@
 
 namespace Symfony\Component\Routing\Tests\Loader;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Routing\Loader\ObjectRouteLoader;
 use Symfony\Component\Routing\Route;
 use Symfony\Component\Routing\RouteCollection;
 
-class ObjectRouteLoaderTest extends \PHPUnit_Framework_TestCase
+class ObjectRouteLoaderTest extends TestCase
 {
     public function testLoadCallsServiceAndReturnsCollection()
     {

Tests/RouteCollectionBuilderTest.php

@@ -11,12 +11,13 @@
 
 namespace Symfony\Component\Routing\Tests;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Config\Resource\FileResource;
 use Symfony\Component\Routing\Route;
 use Symfony\Component\Routing\RouteCollection;
 use Symfony\Component\Routing\RouteCollectionBuilder;
 
-class RouteCollectionBuilderTest extends \PHPUnit_Framework_TestCase
+class RouteCollectionBuilderTest extends TestCase
 {
     public function testImport()
     {