You cannot complete a test order on a theme that is not active. However, you can preview that theme. Is there something specific you want to test? The checkout will work and use the same shipping rates that you currently have set up.
Please let me know if you have any further questions! I would love to learn more about your business. What type of products do you sell?
Emily | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog