Ahhh I just reported this issue and finally found this.

Apple Developer Support just told me: "If you already have the subscription, the only way to test it would be to provide the code to someone else who does not have an active subscription." Even if you find someone else to test it, if you find a bug post code-redemption, then you'll need to find another person again to confirm that the entire flow works. This is a horrible way to test out a feature...

Same