I'd like to keep the `config` attribute mainly as (loosely) read-only, b/c your feedback earlier that it's confusing [when reading test code].  I put in ``self.sub_stuff`` for caller-use and defined that it wouldn't be touched, outside of test-modules.  That's the reason I used a private ``_sscd`` attribute here (which is kind of klunky).  So I'm not sure what to do here, maybe should we just have an instance attribute ``post_subsubtests`` or something?  I'll look at this more after I work on the docstrings, but if you have an idea, I'm open.