XCTest testing framework

The XCTest library is a common framework for writing unit tests in Swift. Usually, we just have to write the unit tests once and they can be executed across different platforms without rewriting. Each test is organized into an XCTestCase subclass with many different test methods. Each method shall be started with a prefix "test". We can run the tests from a Terminal on Linux or macOS. For Linux, an extra Linux main file with an array containing all available tests is needed. For macOS, XCode CLI tools to execute the tests are required. The XCTest framework is also well integrated into the workflow in XCode. We can use the scheme editor to specify which targets, classes, and methods to include a test, and use the XCode test navigator to run tests and view the results.