Simplify specs on spec/models/project_services/pipelines_email_service_spec.rb

After the addition of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28271, specs on https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/models/project_services/pipelines_email_service_spec.rb are getting a bit hard to follow due to all the possible combinations.

A possible idea to refactor would be to use paramterized tests