Unit testing Task.Delay

DelayService.cs

    public interface IDelayService
    {
        Task Delay(TimeSpan delayDuration, Action continueWith);
    }
    
    internal class DelayService : IDelayService
    {
        public virtual async Task Delay(TimeSpan delayDuration, Action continueWith)
        {
            await Task.Run(async () => { await Task.Delay(delayDuration); })
                .ContinueWith(_ => continueWith())
                .ConfigureAwait(false);
        }
    }
    
    internal class DelayServiceStub : DelayService
    {
        public AutoResetEvent DelayComplete = new AutoResetEvent(false);

        public override async Task Delay(TimeSpan delayDuration, Action continueWith)
        {
            await base.Delay(TimeSpan.Zero, continueWith);
            DelayComplete.Set();
        }
    }