Add spec for enabling shared runners in groups

Add spec for Enable shared runners for this group

We should add a feature spec to confirm that shared runners can be enabled/disabled.

Original discussion

The following discussion from !84165 (merged) should be addressed:

• @syasonik started a discussion: (+1 comment)

  suggestion: It would be nice to go the step further and test the toggle behavior here, instead of only its presence. In this flow, the frontend is sending a request to the backend, which is the prime scenario for feature spec coverage.

  We could test that clicking the toggle updates the setting in the DB, which matches the resultant toggle state in the UI.

  WDYT?