Enable a debugger to be attached to unit tests

Our current `nox`-based approach does not make it easy to attach a debugger to our unit tests. This makes debugging them time-consuming and awkward. Once we figure out a clean solution, we should document it in our Contributor's Guide

Start by inspecting the current nox-based unit-test workflow and the Contributor's Guide. Determine a clean way to attach a debugger to those tests, then document the resulting workflow in the guide; done means contributors can follow it without the current debugging friction.

Written by the indexing model from the issue text.