move PHPUnitExtension and Subscriber into Extension namespace

phpunit.xml.dist

@@ -29,7 +29,7 @@
     </testsuites>
 
     <extensions>
-        <bootstrap class="Cake\TestSuite\Fixture\PHPUnitExtension"/>
+        <bootstrap class="Cake\TestSuite\Fixture\Extension\PHPUnitExtension"/>
     </extensions>
 
     <source>

src/TestSuite/Fixture/PHPUnitExtension.php

@@ -14,9 +14,8 @@ declare(strict_types=1);
  * @since         4.3.0
  * @license       https://opensource.org/licenses/mit-license.php MIT License
  */
-namespace Cake\TestSuite\Fixture;
+namespace Cake\TestSuite\Fixture\Extension;
 
-use Cake\TestSuite\Fixture\PHPUnitSubscriber\PHPUnitStartedSubscriber;
 use PHPUnit\Runner\Extension\Extension;
 use PHPUnit\Runner\Extension\Facade;
 use PHPUnit\Runner\Extension\ParameterCollection;

src/TestSuite/Fixture/PHPUnitSubscriber/PHPUnitStartedSubscriber.php

@@ -14,7 +14,7 @@ declare(strict_types=1);
  * @since         5.0.0
  * @license       https://opensource.org/licenses/mit-license.php MIT License
  */
-namespace Cake\TestSuite\Fixture\PHPUnitSubscriber;
+namespace Cake\TestSuite\Fixture\Extension;
 
 use Cake\Log\Log;
 use Cake\TestSuite\ConnectionHelper;