Add testing/utils.py for use of these tests in job repos

Along the way, I realized that configuration of error_sink doesn't actually work. The auto error sink stuff works, but specifying an error_sink descriptor from config does not.

I spent a lot of time trying to find a nice way to make it work, but I need to pause on this to work on getting this code running in prod. Let's revisit this.

Bug: T328478