From a dev standpoint, the only difference between testing and production is a toggle, change the product SKU from in development true, to false. Functionality doesn’t change.
From Twitch’s point of view, this would be open to abuse, lacks sufficient use case as they’ve provided plenty of ways for you to test yourself, as well as working alongside an affiliate/partner if that’s what you really want to do. To allow devs to use bits would also devalue both their affiliate and partner programs, as anyone can sign up to be an extension dev, so allowing you to spend real bits would hurt their own affiliate/partner programs for no real benefit.