Here's how you can create a hidden one click upsell funnel for testing without showing it to end user:
Step 1: Create a hidden product you can set it to a small amount. Let's call it TEST PRODUCT.
Step 2: Mark TEST PRODUCT's visibility to Hidden
Step 3: Now create a new funnel and add a checkout and Upsell page to it.
Step 4: Add a Rule "Product is TEST Product". This rule will limit the funnel to run only on TEST PRODUCT.
Step 5: Go to Offers and add the TEST PRODUCT to it
Step 6: Go to design and customize the design of this offer. With Steps 1 to 6, you have now set up a funnel that triggers when TEST PRODUCT and also shows TEST PRODUCT as an offer.
Step 7: Go to Global Settings > Gateways and Enable Test Gateway. You are all set!
Step 8: Now go to frontend and add TEST PRODUCT to the cart
Step 9: During checkout you would see TEST Gateway, select it and place an order. You would see the offer like this:
Step 10: Accept the offer and see that confirmation is shown on Thank You page
Step 11: Repeat steps 8 and 9 to trigger funnel again and this time reject the offer.
Important Note: If all works the flow via Test Gateway is complete. It is strongly advised that you do Step 8-10 for each of the gateways. You may have to do a small test transaction which you can refund later from your account. But this would ensure that everything is properly set up.