It is bad forum practice to necro a old post. as then it triggers notifications for everyone on the post
Testing is covered here
But you CANNOT test the JWT in onTransactionComplete as you only get it on a LIVE extension running on the Twitch Website. The JWT itself cannot be tested in the Developer rig.
As per the docs
you should use the setUseLoopBack function to test the compleition of a checkout flow. But you still won’t get a JWT in the onTransactionComplete as that on occurs for “live”