Add group runners test cases

What does this MR do and why?

This change ports the existing test cases that covered Group -> Settings -> CI/CD -> Runners to the new UI at Group -> Runners

Some tests are refactored so they are reused by admin runners feature tests as well.

Related to #352169 (closed)

Screenshots or screen recordings

NA

How to set up and validate locally

NA

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.