Test templates and GitLabCI parser againts each other

## What does this MR do?

Test the available templates against the preprocessor and vice versa

## Are there points in the code the reviewer needs to double check?

The dynamic creation of tests seems a little hacked. Is there a cleaner way?

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added -- Seems unneeded
- [x] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)

See merge request !4898