Resolve "Improve code coverage of app/models/user_highest_role.rb"

What does this MR do and why?

This MR adds missing coverage for the UserHighestRole.allowed_values method as part of Improve code coverage around areas related to p... (&12239).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #435153 (closed)