Add stub file I missed earlier.

tests/test_app/TestApp/Shell/AppShell.php

@@ -0,0 +1,24 @@
+<?php
+/**
+ * CakePHP(tm) : Rapid Development Framework (https://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ * @link          https://cakephp.org CakePHP(tm) Project
+ * @since         3.6.5
+ * @license       https://opensource.org/licenses/mit-license.php MIT License
+ */
+namespace TestApp\Shell;
+
+use Cake\Console\Shell;
+
+/**
+ * Stub class to ensure inclusion in command list.
+ */
+class AppShell extends Shell
+{
+}