Extract customer creation/cleanup

This also ensures that cleanup always runs after all integration tests

MAILPOET-5145

mailpoet/tests/integration/Models/SubscriberCustomFieldTest.php

@@ -77,6 +77,7 @@ class SubscriberCustomFieldTest extends \MailPoetTest {
   }
 
   public function _after() {
+    parent::_after();
     SubscriberCustomField::deleteMany();
   }
 }