Add test case for `first_class_vulnerabilities` feature flag

What does this MR do?

This is a follow up MR which introduces test cases for !28652 (merged) which did not have the test cases due to tight deadline.

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

/closes #213802 (closed) /cc @svedova