Refactor report shared examples with Comparable module

What does this MR do?

Refactor report shared examples with Comparable module, so that report comparison can be tested.

Follow-up MRs

• Code linting, to remove all offenses reported by Rubcop. See comment and other comments. => Done in !206 (merged)
• ComparableReport class, possibly on top of the existing functions. See thread.

What are the relevant issue numbers?

This is related to gitlab-org/gitlab#299503 (closed) since all jq filters of compare_reports.sh are ported to Ruby.

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