Additional test coverage for [COOK-2353] 

We had no unit test coverage for the core bit of functionality introduced in [COOK-2353]. This new coverage ensures `Chef::Provider::Service::Runit#run_action(:enable)` only makes an internal call to `#restart_service` if the embedded run_script template resource is updated AND the restart_on_update attribute is true (which is default).

I also cleaned up the stubbing/mocking a bit in `provider_runit_service_spec.rb`.  This should make creating future test coverage much easier.