Explicitly require runner owners in specs

What does this MR do and why?

🚨 NOTE: There is urgency in getting this MR and !166916 (merged) merged soon, since they unblock batched background migrations that must make it to %17.5 (which is a required stop), so we can finalize them in %17.6 🚨

Ensure that all runner objects are valid models, by enforcing the existence of an owner for project and group runners. Previously, it was usual for brevity to do something like:

let(:runner) { create(:ci_runner, :project) }

This would be successful, even though we weren't implicitly creating a project behind the scenes, and therefore the runner wasn't representing a valid state. We now enforce the requirement that a runner is associated with a group or a project. We lean on the pre-existing :without_projects trait to determine if the runner is supposed not to have projects, and therefore refrain from raising an error in the factory.

Part of #493256 (closed)

Context: !166916 (comment 2130033764)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.