I'm not aware of such example, but I think that building such a simple example should not be difficult. Error boundary catches exceptions thrown during render operations. So the test of custom error boundary component would look like this:

1. Define trigger component that always throws during render. Or if you want to be fancy, it can render correctly initially and then throws during subsequent render after some internal state switch. Be sure not to throw during event as that is not covered by error boundary.
2. Render trigger component inside error boundary, let the trigger component fail
3. Expect that output is rendered (i.e. render does not throw but the error is consumed by error boundary), and the the rendered output is that of error boundary component and not of trigger component.