They'd be created with big red warning signs, installed into browsers and other places by anyone who wants to test, and then publicly revoked every month. Rinse, repeat.
The original comment was to do a fire drill and revoke live certificates to force everybody to participate in the test.
You can have a fire drill and evacuate an office building, but you can't hold a full-scale forest fire/volcano/hurricane drill and evacuate 3.5 million people to test mass evacuation routes, and also you got the edge cases that don't participate in drills, like homebound seniors or hospitals, where disaster strikes.
Do they, actually? I got the impression from the article that little revocation testing of any scale happens. If so, then even a limited test would be useful.