@IBOutlet weak var messageLabel: UILabel!

@IBOutlet weak var activityIndicator: UIActivityIndicatorView!

override func viewDidLoad() {
    super.viewDidLoad()

    messageLabel.text = ""
}

@IBAction func startTestButtonPress(_ sender: UIButton) {

    messageLabel.text = "Starting test"

    activityIndicator.startAnimating()

    DispatchQueue.global(qos: .userInitiated).async { [weak self] in

        Thread.sleep(until: Date(timeIntervalSinceNow: 5.0))

        DispatchQueue.main.async {

            self?.activityIndicator.stopAnimating()

            self?.messageLabel.text = "Test completed"
        }
    }
}