Refactor spec/policies/project_policy_spec.rb to minimize the diff with EE
What does this MR do?
CE MR for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2953.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
CE MR for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2953.