Replace downstream pipelines with integration-test for Brakeman

What does this MR do?

• Replaces ruby test downstream pipelines with the integration-test approach.

What are the relevant issue numbers?

• Improve SAST analyzer tests (gitlab-org/gitlab#366852 - closed)

Does this MR meet the acceptance criteria?

• Changelog entry added Not applicable on developer-facing changes
• 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
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer