Navigating to the details

We will implement the navigation within the app using the AppCoordinator class. Follow these steps to implement navigation to the details of to-do items:

1. Add the following test method to AppCoordinatorTests:

   // AppCoordinatorTests.swift
   func test_selectToDoItem_pushesDetails() throws {
     let dummyViewController = UIViewController()
     let item = ToDoItem(title: "dummy title")
     sut.selectToDoItem(dummyViewController, item: item)
     let detail = try XCTUnwrap(
       navigationControllerMock.lastPushedViewController
       as? ToDoItemDetailsViewController)
     XCTAssertEqual(detail.toDoItem, item)
   }

In this test, we execute the delegate method and assert that an instance of ToDoItemDetailsViewController is pushed to the navigation stack, and that its toDoItem is the item we used in the delegate method call.

Run the tests to confirm that this new test...