Add test for previous commit.

tests/TestCase/Form/FormTest.php

@@ -16,6 +16,8 @@ namespace Cake\Test\TestCase\Form;
 
 use Cake\Form\Form;
 use Cake\TestSuite\TestCase;
+use TestApp\Form\AppForm;
+use TestApp\Form\FormSchema;
 
 /**
  * Form test case.
@@ -39,6 +41,9 @@ class FormTest extends TestCase
         $schema = $this->getMockBuilder('Cake\Form\Schema')->getMock();
         $this->assertSame($schema, $form->schema($schema));
         $this->assertSame($schema, $form->schema());
+
+        $form = new AppForm();
+        $this->assertInstanceOf(FormSchema::class, $form->schema());
     }
 
     /**

tests/test_app/TestApp/Form/AppForm.php

@@ -0,0 +1,22 @@
+<?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         3.5.13
+ * @license       https://opensource.org/licenses/mit-license.php MIT License
+ */
+namespace TestApp\Form;
+
+use Cake\Form\Form;
+
+class AppForm extends Form
+{
+    protected $_schemaClass = FormSchema::class;
+}

tests/test_app/TestApp/Form/FormSchema.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         3.5.13
+ * @license       https://opensource.org/licenses/mit-license.php MIT License
+ */
+namespace TestApp\Form;
+
+use Cake\Form\Schema;
+
+/**
+ * Contains the schema information for Form instances.
+ */
+class FormSchema extends Schema
+{
+}