Merge downstream into test stage

What does this MR do?

Merge the downstream stage into the test stage, to run the pipeline faster.

See gitlab-org/security-products/analyzers/retire.js!26 (closed) to see the pipeline it generates:

What are the relevant issue numbers?

None.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• [-] Documentation created/updated for GitLab EE, if necessary
• [-] Documentation created/updated for this project, if necessary
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Job definition updated, if necessary
• [-] Conforms to the Go guidelines
• [-] Security reports checked/validated by reviewer