This code allows for testing UNNotificationCenter and UNNotificationSettings

CountdownTests.swift

func testNotifications() {
        // map all authorizationStatus with expected Result
        let authorizationStatusMap: [UNAuthorizationStatus: Int] = [.authorized: 1, .denied: 0, .notDetermined: 0, .provisional: 1]
        UNNotificationSettings.swizzleAuthorizationStatus()
        
        authorizationStatusMap.forEach { (key: UNAuthorizationStatus, value: Int) in
            UNNotificationSettings.fakeAuthorizationStatus = key
            
            let mockCenter = UserNotificationCenterMock()
            let mockCoder = MockNSCoder()
            mockCenter.settingsCoder = mockCoder
            let mockDelegate = MockCountdownDelegate()
            let defaults = MockUserDefaults()
            let timer = Countdown(delegate: mockDelegate, defaults: defaults, userNotificationCenter: mockCenter)
            
            XCTAssertEqual(mockCenter.pendingNotifications.count, 0)
            
            let mockContent = UNMutableNotificationContent()
            mockContent.title = "title"
            mockContent.body = "body"
            
            timer.startCountdown(with: Date().addingTimeInterval(1), with: mockContent)
            XCTAssertEqual(mockCenter.pendingNotifications.count, value)
            
            timer.skipRunningCountdown()
            XCTAssertEqual(mockCenter.pendingNotifications.count, 0)
        }
    }

extension UNNotificationSettings {
    static var fakeAuthorizationStatus: UNAuthorizationStatus = .authorized
    
    static func swizzleAuthorizationStatus() {
        let originalMethod = class_getInstanceMethod(self, #selector(getter: authorizationStatus))!
        let swizzledMethod = class_getInstanceMethod(self, #selector(getter: swizzledAuthorizationStatus))!
        
        method_exchangeImplementations(originalMethod, swizzledMethod)
    }
    
    @objc var swizzledAuthorizationStatus: UNAuthorizationStatus {
        return Self.fakeAuthorizationStatus
    }
}

class UserNotificationCenterMock: UserNotificationCenter {
    
    var pendingNotifications = [UNNotificationRequest]()
    var settingsCoder = MockNSCoder()
    
    func getNotificationSettings(completionHandler: @escaping (UNNotificationSettings) -> Void) {
        let settings = UNNotificationSettings(coder: settingsCoder)!
        completionHandler(settings)
    }
    
    func removeAllPendingNotificationRequests() {
        pendingNotifications.removeAll()
    }
    
    func add(_ request: UNNotificationRequest, withCompletionHandler completionHandler: ((Error?) -> Void)?) {
        pendingNotifications.append(request)
        completionHandler?(nil)
    }
}

class MockNSCoder: NSCoder {
    var authorizationStatus = UNAuthorizationStatus.authorized.rawValue
    
    override func decodeInt64(forKey key: String) -> Int64 {
        return Int64(authorizationStatus)
    }
    
    override func decodeBool(forKey key: String) -> Bool {
        return true
    }
}

UserNotificationProtocol.swift

protocol UserNotificationCenter {
    func getNotificationSettings(completionHandler: @escaping (UNNotificationSettings) -> Void)
    func removeAllPendingNotificationRequests()
    func add(_ request: UNNotificationRequest, withCompletionHandler completionHandler: ((Error?) -> Void)?)
}

extension UNUserNotificationCenter: UserNotificationCenter {}