Extract simple workers common code into a base class [PREMIUM-4]

tests/unit/Cron/Workers/KeyCheck/KeyCheckWorkerMockImplementation.php

@@ -0,0 +1,12 @@
+<?php
+namespace MailPoet\Cron\Workers\KeyCheck;
+
+if(!defined('ABSPATH')) exit;
+
+class MockKeyCheckWorker extends KeyCheckWorker {
+  const TASK_TYPE = 'mock_key_check_worker';
+
+  function checkKey() {
+    return array('code' => 12345); // bogus code
+  }
+}