Refactor admin runners tests cover admin and groups

Related to the release of this new feature: #336405 (closed).

Description

Create a set of feature specs for the Group runners UI.

URL: http://gdk.test:3000/groups/<group>/-/runners

Implementation Details

Our Admin runners UI and our Group runners UI share many features for different roles:

Admin (/admin/runners)	Group (/groups/<group>/-/runners)

The admin tests are at spec/features/admin/admin_runners_spec.rb. These same tests could be reused/shared with the group runners.