Need to exec something on .done() without having to write a custom reporter.

The built-in reporters work great. But I need to be able to run something on `.done()` and I don't see any way of doing that other than to write my own reporter. It would be painful to have to write and maintain a custom reporter just for the sake of adding in some "execute this callback when all done" functionality, when I don't want to change any other behavior.

Can you add in the ability to do this?

Start by reading the built-in reporter flow and the `.done()` entry point to see where completion is handled. Define the callback behavior around that completion point and verify that it runs after all tests finish without changing existing reporter behavior.

Written by the indexing model from the issue text.