When you create tests for your application, you can choose to store them within the application, or you can keep them contained to a standalone test application. 

Creating a standalone test application allows you to create a separation between what you intend on releasing to your customers, and what you use only internally.

The test application is created in the same fashion that a normal application is created, but there are a few key points to remember:

• The test application needs to have its own unique ID, name, and object range.
• The test application needs a dependency on the application it is testing so that you can directly reference the entities from the test application.
• If you intend on referencing any of the default test libraries from Microsoft, you need to add the test property to app.json.