Exclude test files from security scans

What does this MR do?

Results from *_test.go files are showing in the Vulnerability Report. We should exclude these files.

Note: I tried to make this change more globally with the CI Template the Secure stage uses, but couldn't get that to work.

This change does not need to be a separate release, so there's no changelog entry.

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