I'd leave the default (Settings > All Settings > Software > Patch Management > Windows Settings > Windows Patch Remediation Settings) as your test PC. You'll have to change the target of each Patch Policy individually (Manage > Policies > Software > Patch Management > Software Update Policies).
You could create a Target called "My current patch target" and make that the default. Put your test PC in it and include the "Windows Computers with Software Update Plug-In Installed" filter too. Then, when you want to test a new patch, edit the Target to remove the "Windows Computers with Software Update Plug-In Installed" filter and add it back when you're satisfied with the test. It would mean that your production machines wouldn't be covered by patch policies when you're testing but, depending on your patch schedules that wouldn't be a problem.
I think you have to create and edit targets from within a Policy - they're not available anywhere else.