public void Concert_BuyTickets_TicketsBuySuccessfull()
        {
            Concert concert = CreateConcert(3);

            concert.BuyTickets(2, 3);

            Assert.True(concert.BoughtTickets.Quantity == 2);
        }
        public void Concert_BuyTickets_MaximumNumnberOfTicketsReached()
        {
            Concert concert = CreateConcert(3);

            Assert.Throws <MaximumNumberOfTicketsReached>(() => concert.BuyTickets(4, 3));
        }