Run global nav link test in review apps

This splits the global nav test out of the standard tests, so that it can run on review apps. This way if you delete a doc in gitlab (or another project) and run a review app, the review app pipeline will fail, giving you an early warning that you are about to cause a broken default branch in gitlab-docs.

Related to #1306 (closed)

The rules are identical to the .rules_site_tests, except the when: never is removed from review app pipelines (if: '$CI_PIPELINE_SOURCE == "pipeline" || $CI_PIPELINE_SOURCE == "trigger"')