chore: Add tests for Run command

What does this MR do?

We previously had no unit tests around the Run command, which is a fairly critical entrypoint for all our scanners. This commit adds some basic tests for ensuring happy paths, errors for subcommands are handled properly, and serialization checks the full artifact path not just the artifact name.

What are the relevant issue numbers?

See related discussion !20 (comment 725557267)

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