Use `fake` Driver in e2e tests

[acpana]

With multi-engine support in (https://github.com/open-policy-agent/frameworks/pull/293), our e2e testing harness relies on creating a rego driver only. That is fine for testing the rego driver but we should use the fake Driver more in our e2es as per: https://github.com/open-policy-agent/frameworks/pull/283#discussion_r1152619750 :

We should move away from using Rego to using the fake driver in tests. This will reduce randomness and allow us to do things like inject specific stats responses, error responses, etc.