Create relative-url test job against CNG environment

Relevant discussions

• #3706 (comment 2644022904)
• gitlab-org/gitlab#439943 (comment 2509736376)

Problem

The relative-url test job is effective at finding bugs, but we should shift it left.

We have a relative-url job that runs against omnibus instances on master, but to get faster feedback we should utilise CNG, as the tests against CNG are blocking and run in MRs.

Considerations

• perhaps it's best to run a subset of tests rather than the entire suite?
• we may be able to remove/reduce the relative-url from the omnibus e2e test pipeline also
• The relative-url will only execute the smoke suite
• Configuration documentation https://docs.gitlab.com/charts/charts/globals/#configure-a-relative-url-root