Improve specs for Ci::ProcessBuildService

These specs are quote complex. Is there a way to use parametrized rspec block to simplify it?

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21767#note_106802968

Can we avoid magic local variables b / h?

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21767#note_106803824