How to run PyBuilder unit tests in sub directories

Question

Has anyone worked out how to run a pybuilder unit test, using the default configuration, whereby the _tests.py file is in a sub directory(s).

I can get the test to run when in the default successfully.

OK

src/unittest/python/my_tests.py

NOT OK

src/unittest/python/a/b/c/my_tests.py

I've tried with and without the __init__.py files in the unit test directories, no joy.