Test purchases can be made in your shopping cart by setting up a "comp" (complimentary) payment option temporarily.
1.To do this choose "Shopping Cart" from the 'Commerce' section of the control panel.
2.Click on "Add/Edit Payment Methods".
3.Click the "Add New Payment Method" button.
4. For "Payment Method Code:" enter: comp
5. For "Payment Method Description:" enter: comp
6. Set the status to active and press Add Payment Method.
Items in your cart will have comp listed on the checkout form in addition to Visa/Mastercard etc.
Enter any dummy card number (1111222233334444) etc to test the purchase.
IMPORTANT: make the comp payment method inactive when you are not testing or else anyone can choose this to make a test purchase.