Add test to check for `@project_runners` in `ci_cd_controller_spec.rb`

The following discussion from !104597 (merged) should be addressed:

• @ahmed.hemdan started a discussion:

  suggestion (non-blocking): I realise this is likely outside of the scope of this merge request, but shall we perhaps add another context for @project_runners which are also rendered in the page similar to this context for @assignable_runners?