Fix unit test for Template_Registry

[MAILPOET-6453]

packages/php/email-editor/tests/integration/Engine/Templates/Templates_Test.php

@@ -51,7 +51,7 @@ class Templates_Test extends \MailPoetTest {
 	 */
 	public function testItTriggersActionForRegisteringTemplates(): void {
 		$trigger_check = false;
-		add_action(
+		add_filter(
 			'mailpoet_email_editor_register_templates',
 			function ( $registry ) use ( &$trigger_check ) {
 				$trigger_check = true;