The _do_discovery() method of unittest.TestProgram:

    def _do_discovery(self, argv, Loader=loader.TestLoader):

(from http://hg.python.org/cpython/file/2cf89e2e6247/Lib/unittest/main.py#l222 )

should be using self.testLoader instead of loader.TestLoader.  It's not consistent for some parts of TestProgram to use loader.TestLoader and other parts to use the testLoader passed in via the constructor.