I think this one only didn't work with discovery by accident: the only change necessary to make discovery pass was to make _TestPoll match all other _Test* classes by inheriting from BaseTestCase instead of unittest.TestCase.  The remainder of the changes are to eliminate test_main by converting it instead to setUpModule.  setUpModule is, I think, necessary on this one due to a failing test in WithProcessesTestLogging when the initial call to multiprocessing.get_logger().setLevel() (originally in test_main) is moved to top-level code.