Extract customer creation/cleanup

This also ensures that cleanup always runs after all integration tests

MAILPOET-5145

mailpoet/tests/unit/API/JSON/ErrorResponseTest.php

@@ -40,6 +40,7 @@ class ErrorResponseTest extends \MailPoetUnitTest {
   }
 
   public function _after() {
+    parent::_after();
     WPFunctions::set(new WPFunctions);
   }
 }