> The service needs to bill "credits"

When talking about e2e tests, it matters where the e's are. If they are at the public interface of "the service", then, indeed, you'd test against "credits".

But ideally, there'd be e2e tests that test the consumers' experience, where the 'e's are the web interface, emails, CLI and other things consumers click on, read, enter etc. This is where a "test currency" makes sense.