minor #21740 Fix missing namespace in test (chalasr)

This PR was merged into the 2.7 branch.

Discussion
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Fix missing namespace in test

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Tests pass?   | yes

Commits
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1e9ca7b Fix missing namespace in AddConstraintValidatorPassTest

src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/Compiler/AddConstraintValidatorsPassTest.php

@@ -9,6 +9,8 @@
  * file that was distributed with this source code.
  */
 
+namespace Symfony\Bundle\FrameworkBundle\Tests\DependencyInjection\Compiler;
+
 use PHPUnit\Framework\TestCase;
 use Symfony\Bundle\FrameworkBundle\DependencyInjection\Compiler\AddConstraintValidatorsPass;