I'm not sure I fully understand what you're trying to do with your test runner here, but am I correct that you are trying to execute something before an instance enters play mode? If that's the case you should have some options.
The extra instances are just stripped-down editors, so they'll still run any editor script you have set up in your project. For example a callback registered with playModeStateChanged will still execute in your instances. So you could check for the ExitingEditMode state and have some code run right before your instance enters play mode.
If you're using scenarios, you can also go to the "Play Mode Status Window" (from the drop-down where you select your scenario) and switch your instances to "Manual Control" and activate them manually. This will give you access to the instances while you are still in edit mode. The instances will also survive exiting play mode in that state. Unfortunately in terms of UI you're still pretty limited by what you can do in those extra instances (e.g. no inspector in edit mode) so this might not be that useful. It does make iteration faster though.