Move fixtures from JSON to go structs

What does this MR do?

We currently use JSON expectation files in this project, however, these files present a number of problems, as discussed here which require workarounds.

This MR replaces the JSON expectation files with Go structs so they can be compared directly, thereby simplifying the tests by removing the need for workarounds.

What are the relevant issue numbers?

N/A

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
• Ensure the report version matches the equivalent schema version
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer