Presenting two sections

As we have already refactored to a diffable data source, supporting two sections in the table view is quite easy. Follow these steps to implement two sections:

1. As always, we need to start with a failing test. Add the following test to ToDoItemsListViewControllerTests:

   // ToDoItemsListViewControllerTests.swift
   func test_numberOfSections_shouldReturnTwo() {
     var doneItem = ToDoItem(title: "dummy 2")
     doneItem.done = true
     toDoItemStoreMock.itemPublisher
       .send([ToDoItem(title: "dummy 1"),
              doneItem])
     let result = sut.tableView.numberOfSections
     XCTAssertEqual(result, 2)
   }

In this test, we set a to-do item and a done item to the table view using toDoItemStoreMock. The name of the test method should also include what the preconditions of the tests are. We use a shorter name in the book because...