Swift2.3で記述したUserNotificationコード(下記)でuserNotificationCenterが呼ばれません。
基本的に同等(と思われる)Swift3では動作します。基本的というのは、Swift2.3 と Swift3 でシンタックスが異なり、Xcodeの補完に従って変更しています。ここで誤っているのかもしれませんが、、、

import UIKit
import UserNotifications

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate, UNUserNotificationCenterDelegate {

    var window: UIWindow?

    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
        // Override point for customization after application launch.

        if #available(iOS 10.0, *) {
            // iOS 10
            let center = UNUserNotificationCenter.currentNotificationCenter()

            center.requestAuthorizationWithOptions([.Badge, .Sound, .Alert], completionHandler: { (granted, error) in
                if error != nil {
                    return
                }

                if granted {
                    debugPrint("通知許可")

                    center.delegate = self

                    let content = UNMutableNotificationContent()
                    content.title = "Title"
                    content.subtitle = "Subtitle" // 新登場！
                    content.body = "Body"
                    content.sound = UNNotificationSound.defaultSound()

                    // 5秒後に発火
                    let trigger = UNTimeIntervalNotificationTrigger(timeInterval: 5, repeats: false)
                    let request = UNNotificationRequest(identifier: "FiveSecond",
                                                        content: content,
                                                        trigger: trigger)

                    // ローカル通知予約
                    center.addNotificationRequest(request, withCompletionHandler: nil)

                    center.getPendingNotificationRequestsWithCompletionHandler { (requests: [UNNotificationRequest]) in
                        print(">>>>> \(requests)")
                    }

                } else {
                    debugPrint("通知拒否")
                }
            })

        } else {
            // <= iOS 9
//            let settings = UIUserNotificationSettings(forTypes: [.Badge, .Sound, .Alert], categories: nil)
//            UIApplication.shared.registerUserNotificationSettings(settings)
        }

        return true
    }

    @available(iOS 10.0, *)
    func userNotificationCenter(center: UNUserNotificationCenter,
                                willPresent notification: UNNotification,
                                withCompletionHandler completionHandler: (UNNotificationPresentationOptions) -> Void) {

        // ...
        let identifier = notification.request.identifier
        debugPrint("userNotificationCenter \(identifier)")
   }
}