I don't see a problem with accepting a test credit card. Once your free trial is over, then they'll email you a message saying your card can't be charged and you'll be expected to update it.
The validation that they are doing when you enter the card is just Luhn validation.§
This is basically just to ensure that the numbers entered into the field "at least have the possibility of being a valid credit card". No processing of the card is being done at this stage.