Extract customer creation/cleanup

This also ensures that cleanup always runs after all integration tests

MAILPOET-5145

mailpoet/tests/integration/Newsletter/ViewInBrowser/ViewInBrowserControllerTest.php

@@ -237,6 +237,7 @@ class ViewInBrowserControllerTest extends \MailPoetTest {
   }
 
   public function _after() {
+    parent::_after();
     // reset WP user role
     $wpUser = wp_get_current_user();
     $wpUser->add_role('administrator');