Resolve #3254472 "Update tests to work with Drupal 9"

Closes #3254472

tests/src/Functional/ModuleInstallUninstallWebTest.php

 <?php
 
-namespace Drupal\flysystem_s3\Tests;
+namespace Drupal\Tests\flysystem_s3\Functional;
 
-use Drupal\flysystem\Tests\ModuleInstallUninstallWebTest as Base;
+use Drupal\Tests\flysystem\Functional\ModuleInstallUninstallWebTest as Base;
 
 /**
  * Tests module installation and uninstallation.