Replace fake client with envtest in tests

There are some cases where the fake client doesn't reflect the behaviour of a real API server. This bug contains one example: https://github.com/cryostatio/cryostat-operator/issues/447#issuecomment-1242491587. We already spin up envtest as part of our test suite, we just don't use it. This is closer to a real API server than the fake client represents. We should switch to using it.