Add PG vars to Test.gitlab-ci.yml

What does this MR do?

Test template fails if include outside of AutoDevOps template because of this 2 missing variables. Adding those will make test template usable (via include) in any gitlab-ci.yml file.

For example gitlab-org/5-minute-production-app/examples/rails-app@308a1d07. Without those variables it uses some random username which cause https://gitlab.com/gitlab-org/5-minute-production-app/examples/rails-app/-/jobs/869532870 fail.

Variables copied from parent https://gitlab.com/gitlab-org/gitlab/-/blob/4d03aa040fc8c2564a8e23a5dfec0b9c006512ee/lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml#L50-51

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content