Extract customer creation/cleanup

This also ensures that cleanup always runs after all integration tests

MAILPOET-5145

mailpoet/tests/integration/Cron/Workers/KeyCheck/SendingServiceKeyCheckTest.php

@@ -117,6 +117,7 @@ class SendingServiceKeyCheckTest extends \MailPoetTest {
   }
 
   public function _after() {
+    parent::_after();
     $this->diContainer->get(SettingsRepository::class)->truncate();
   }
 }