There are 3 distinct testing environments: Production (App Store), TestFlight (Production Sandbox), and Sandbox (Developer Builds). Each behaves slightly differently and needs to be tested independently.
The developer sandbox is the first line of defense. Make sure you understand the quirks and limitations during development to save time when you move on to production testing.
For testing purposes, the subscription will automatically renew up to 6 times per account. The actual number of renewals is random. After at most 6 renewals, the subscription will automatically stop renewing. For more specifics about subscription length click on the "read more here" button at the bottom of this article.
While not especially helpful with beta testers, you should definitely spend some time testing in the production sandbox before shipping. TestFlight behaves like the sandbox but uses production App Store accounts.
There are a few tricks to testing in production even before a release hits the App Store, but you’ll also want to keep testing live on the App Store as the app is updated.