public void ToDigitsOnly_WithNonDigits_ReturnsNormalized(string cardNumber, string expected)
        {
            var result = CreditCardNumberFormatter.ToDigitsOnly(cardNumber);

            Assert.Equal(expected, result);
        }
        public void ToDigitsOnly_WhenValid_ReturnsUnchanged(string cardNumber)
        {
            var result = CreditCardNumberFormatter.ToDigitsOnly(cardNumber);

            Assert.Equal(cardNumber, result);
        }