spec/controllers/projects/jobs_controller_spec.rb is over-testing serialized responses

The following discussion from gitlab-ce!23518 should be addressed:

• @rspeicher started a discussion:

  I know a lot of this was pre-existing but we might be over-testing here. It seems like this is essentially re-testing the serializer, which we already seem to test pretty thoroughly. I think all the controller spec needs to do is verify we render with a serializer.

  That said, fixing that is probably outside the scope of this MR.