Add fixture I missed.

tests/test_app/Plugin/TestPlugin/tests/Fixture/Blog/CommentsFixture.php

@@ -0,0 +1,43 @@
+<?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         3.4.9
+ * @license       https://opensource.org/licenses/mit-license.php MIT License
+ */
+namespace TestPlugin\Test\Fixture\Blog;
+
+use Cake\TestSuite\Fixture\TestFixture;
+
+/**
+ * Plugin comments fixture.
+ */
+class CommentsFixture extends TestFixture
+{
+
+    /**
+     * fields property
+     *
+     * @var array
+     */
+    public $fields = [
+        'id' => ['type' => 'integer'],
+        'article_id' => ['type' => 'integer', 'null' => false],
+        'comment' => 'text',
+        '_constraints' => ['primary' => ['type' => 'primary', 'columns' => ['id']]]
+    ];
+
+    /**
+     * records property
+     *
+     * @var array
+     */
+    public $records = [];
+}