No you dont need to change any code. Its all handled from backend.
Generally for development purpose we use Cash on delivery or Check/Money Order payment method.
So you just need to enable these method from backend.
For Testing Credit Cards enable 'Saved CC' Payment Method and during checkout use test credit card type VISA
, card number 4111111111111111
with any date of future for Expiration Date and any cvv number
Additionally for proper payment gateway integration You will need to purchase payment gateway which will provide Magento Module along with it. This Payment Method will have 2 modes test and live that are configurable from banckend. So with test mode you can continue development and testing.
Similarly with Paypal. Yo can setup Sandbox test account. Module is already there.