> Might it be simpler to run doctest over the rst file from the relevant unittest?  (Perhaps with help from test.support.)

If I understand correctly, do you mean for example that Lib/test/test_textwrap.py could be responsible for loading the doctests in Doc/library/textwrap.rst?

One problem with this approach is that the *.rst files are not in one-to-one correspondence with unittest files in Lib/test, etc.  For example, there are folders of *.rst files that may not correspond to any unit test files (e.g. the tutorial folder).  It also introduces more coupling because the unit tests would now have to know about the existence of the Doc/*.rst files (though this could be a nice addition as it would let you run the doctests for a module along with its unit tests if you wanted to run tests only for a particular module).

Or did you mean something else?