Thank you Macky and George.
As with all builds released through TestFlight, there are no real-money transactions that can occur. In fact, if you try to subscribe while running a TestFlight build you will get a prompt like the following:
"Beta testers aren't charged for this In-App purchase, and it will only be available during testing."
So you will have to wait until this version is released on the App Store, and then actually download it from the App Store, if you want to purchase the yearly subscription. We will really appreciate it if you do ☺️
Regarding running a single subscription on multiple devices, yes this is definitely possible.
Please let us know if you have any more questions.