Simulate them on a test course? There are absolutely places with street lights and everything that you could test something. Hell since they don’t need to work you can just have some put up in a parking lot to test with. Who cares.

You don’t need to wait for a city blackout to actually test this kind of scenario.

The thing still has cameras. And LIDAR. It should be fully capable of pulling over on its own safely. Why would not having a traffic light prevent that?

Humans are expected to negotiate this. The robots should be too. That’s part of driving. If the lights fail, the law tells you what you’re supposed to do. And it is not stop the intersection.