PubSub: Add more system tests covering various RBAC-related scenarios

[plamut]

A regression introduced not so long ago could have been prevented if we had more RBAC-related system tests. A system test covering the fix was added in googleapis/google-cloud-python#9507.

Since PubSub backend defines several different user roles, system tests must be added to cover at least the most common use case scenarios that can be affected by the user role used in the session.