public void Single_Source_must_produce_element()
{
var p = Source.Single(1).RunWith(Sink.AsPublisher<int>(false), Materializer);
var c = TestSubscriber.CreateManualProbe<int>(this);
p.Subscribe(c);
var sub = c.ExpectSubscription();
sub.Request(1);
c.ExpectNext(1);
c.ExpectComplete();
}