[Validator] Fix the namespace of RegexTest

Author: dunglas

Tests/Constraints/RegexTest.php

@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-namespace Constraints;
+namespace Symfony\Component\Validator\Tests\Constraints;
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Validator\Constraints\Regex;