T-shirt test

Note that this flow requires the presence of the "review" checkout step. In case
no "review" step is configured, it's possible to output the Smart payment buttons
using the "commerce_paypal" service. The payment is initiated from the checkout "review" step. Once the payment is approved on PayPal, the payment is created on in onReturn() and the customer is redirected to the next checkout step (usually "payment" which is skipped because the order is already paid).