Test variable DS_EXCLUDED_PATHS

What does this MR do?

This MR adds a ds-excluded-paths job which tests the DS_EXCLUDED_PATHS variable to ensure that the given path is ignored and results in no vulnerabilities in the expected report.

This test is sufficient to show that the DS_EXCLUDED_PATHS variable works as expected, since the go-modules-qa job covers the case where nothing has been excluded and vulnerabilities exist as expected.

A changelog entry isn't necessary since there are no behavioural changes

What are the relevant issue numbers?

gitlab-org/gitlab#195866 (closed)

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
  • 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