Move current unit tests to integration tests

tests/integration/Cron/Workers/SimpleWorkerMockImplementation.php

@@ -0,0 +1,12 @@
+<?php
+namespace MailPoet\Cron\Workers;
+
+if(!defined('ABSPATH')) exit;
+
+class SimpleWorkerMockImplementation extends SimpleWorker {
+  const TASK_TYPE = 'mock_simple_worker';
+
+  function init() {
+    // to be mocked
+  }
+}