When you write an app with a unit test target, the code in your app needs to be made available to that module in order to be tested. By default, only entities marked as open or public are accessible to other modules. However, a unit test target can access any internal entity, if you mark the import declaration for a product module with the @testable attribute and compile that product module with testing enabled.
ユニットテストを考慮してアクセス制御が設計されてるのは良いね