private IEnumerable<int> Scan(Source<int, NotUsed> source, TimeSpan? duration = null)
{
duration = duration ?? TimeSpan.FromSeconds(5);
var t = source.Scan(0, (i, i1) => i + i1).RunAggregate(new List<int>(), (list, i) =>
{
list.Add(i);
return list;
}, Materializer);
t.Wait(duration.Value).Should().BeTrue();
return t.Result;
}