public void ShouldResetDisplayWhenBuying()
        {
            _ps.AddPayment(25);
            Assert.That(_ps.ReadDisplay(), Is.EqualTo(1010));
            IReceipt receipt = _ps.Buy();

            Assert.That(_ps.ReadDisplay(), Is.EqualTo(1000));
            Assert.That(receipt.Value(), Is.EqualTo(10));
        }
        public void ShouldProduceValidReceiptWhenBuying(int[] coins, int minutes)
        {
            foreach (var coin in coins)
            {
                _ps.AddPayment(coin);
            }
            IReceipt receipt = _ps.Buy();

            Assert.That(receipt, Is.Not.Null);
            Assert.That(receipt.Value(), Is.EqualTo(minutes));
        }