Fold `PassValidationTests._test_lease_operation_fails_without_passes` into its caller.

Created by: tomprince

It was originally factored out since there were both add_lease and renew_lease methods. However, since tahoe-lafs 1.16, the client no longer defines renew_lease, and we removed it from our protocol in #231. Given that there is now only one caller, we can remove the factoring.

I ran into this, while working out how to add tests for tracking spending.