[Refactoring proposal] Test app structure refactoring

首先阅读 test/fixtures/active_record.rb 和 test_helper.rb，梳理其中包含的 schema、模型、控制器、资源、路由、配置和 monkey-patches。将每种组件类型提取到提议的测试文件中，并从 test_helper.rb 中引入这些文件。当测试应用保持其行为不变，且其结构与提议的布局一致时，即视为完成。

This issue is a (choose one):

• Problem/bug report.
• Feature request.
• Refactor proposal
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

Currently, we put all kind of components of our test app in side test/fixtures/active_record.rb. It contains schema, models, controllers and resources. The file’s name is quite misleading since it contains more than active_record related stuffs. And it’s a 2000 lines file, a little bit too long.

And in the meantime, test_helper.rb has the routes, test configurations, monkey-patches and test app configurations.

As a new contributor, I need to spend more time to figure out the test setup under this structure.

So I suggest we can do a small refactoring, to extract and put different types of components into their own files. Like:

test 
 |——— schema.rb # for schema
 |——— models.rb # for models
 |——— controllers.rb # for controllers
 |——— resources.rb # for resources
 |——— routes.rb # for routes
 |——— app.rb # for app configs
 |——— test_helper.rb # test configs, monkey-patchings and requires

Now we can just include these component files inside test_helper.rb and it’ll be more easier for new contributors to understand the test app.