refactor: Rename `Issue` struct to `Vulnerability`

What does this MR do?

Renames Issue struct to Vulnerability. This is purely a refactoring change that we can carry out now that we have split issue.go from common, making this change opt-in for anyone seeking to switch dependencies

Also introduces testify to replace basic assertions

What are the relevant issue numbers?

Relates to gitlab-org/gitlab#8398 (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