Group visibility levels: Fix rubocop issues from visibility helper test

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What does this MR do and why?

• Related to #455668 (closed)
• Fix rubocop issues / todos
• Follow-up MR for Group visibility levels: Apply best practices t... (!153801 - merged)

🛠 with ❤ at Siemens

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Screenshots or screen recordings

Only backend test changes. No behavioral change

How to set up and validate locally

1. Run affected test

bundle exec rspec  ./spec/helpers/visibility_level_helper_spec.rb