Release testing instructions for WC Payments 5.6.0

Ensure WCPay JS loaded on Checkout page for zero order totals PR #5423

Testing steps

Test 1:

• Add an item to cart.
• Create a 100% discount coupon and add it to the cart, make sure cart total is 0
• Navigate to checkout page, and remove the coupon.
• Without refreshing the page, add a payment method and place the order

Test 2:

• Test that it works for all types of checkout (UPE, non-UPE, Blocks, Pay for Order etc) and other pages with Credit Card form.
• Test that there are no regression on normal checkout.