I know of no payment service provider that offers support for testing that is good for fairly normal 2020 software development processes. The standard seems to be offering one separate sandbox environment along with your production one, which you can use to experiment without literally making charges/refunds/etc, and possibly a set of specific credentials you can use to simulate various outcomes such as a declined card or insufficient funds. I know of none that offers enough for stronger safeguards such as automated integration testing as part of a CI system, nor additional sandbox environments so you can for example test a new integration independently without disrupting your main test/staging system that is integrated with the sandbox you get automatically.

Just to extend the OP's anecdote, we also have someone signed up for real with every payment method we offer, though more recently we've been looking at moving away from these kinds of payment service entirely for this among other reasons.