Add test for default CI variables coming form rails
Overview
GitLab rails creates a set of predefined variables that the runner exposes to the user. However, we are not testing or preventing the runner from overwriting these keys. This can be a problem like !2342 (comment 408630960) where we would have accidentally set CI_JOB_STAGE
to something else.
Proposal
Prevent the developer from doing such mistakes by adding test coverage and introducing some reserved variable keys.