[fix] Check for test credit card numbers before attempting validate a…

[theogravity]

The code uses the numbers out of https://www.paypalobjects.com/en_US/vhelp/paypalmanager_help/credit_card_numbers.htm, but some of them do not pass the validation.

1. I added in a unit test to test each test credit card against the validator.
2. I added a simple check in the validator to look for a test card first before running an algorithm.

References #3

[Swaagie]

Thanks for taking the time to contribute, but this cannot be merged in its current state. The idea of the validator is to validate defensively in production, such that only valid (non-test) numbers are accepted. That said adding a env=development switch (or similar) that works both server and client side this would be a nice feature to allow easy testing :)

[steven10172]

Adding a switch to allow test and valid numbers would be a nice feature to have. I might look into creating a PR over the next couple of weeks.