Add SettingsController cache reset to integration tests

[MAILPOET-1757]

tests/integration/Newsletter/Editor/PostContentManagerTest.php

@@ -8,6 +8,7 @@ use MailPoet\WP\Functions as WPFunctions;
 class PostContentManagerTest extends \MailPoetTest {
 
   function _before() {
+    parent::_before();
     $this->post_content = new PostContentManager();
   }
 
@@ -171,4 +172,4 @@ class PostContentManagerTest extends \MailPoetTest {
 
   function _after() {
   }
-}
+}