Hi @lclu,
Short answer: You probably want to use your debug environment.
@jshaffstall is right: on the Free plan you can’t create staging environments for testing, so if your production deployment is on master then that’s where your global uplink key points. However, you can connect uplinks to your personal debug environment, which runs whatever branch you’ve got checked out in the editor.
Also, if you’re looking for hints on TDD with Anvil, this post from me might also be useful!