The regression was introduced by:

https://gitlab.com/gitlab-org/gitlab-ce/commit/1b8f52d9206bdf19c0dde04505c4c0b1cf46cfbe

I did that because there's a test specifying that a shared runner cannot
be enabled, in the API. So I assume that is the case for non-admin, but
admins should be able to do so anyway.

Also added a test to make sure this won't regress again.

Closes #19039