American Express 4-digit CVV

[vlasscontreras]

American Express cards only support 4-digit security codes.

Your credit card CVV code may not be in the same place on every credit card. If you have an American Express® Card, you will find the four-digit credit card CVV on the front.

Source: https://www.americanexpress.com/ca/en/articles/life-with-amex/learn/what-is-credit-card-cvv/

This PR makes the AmericanExpress cards explicitly have 4-digit CVC and adds 2 test assertions.