Issue #19946: appropriately skip new multiprocessing tests

Thanks to Christian Heimes for noting the buildbot failures and
to Zachary Ware for providing the patch to make the new tests
play nice with both other platforms and unittest test discovery

Lib/test/test_multiprocessing_main_handling.py

@@ -130,10 +130,9 @@ def _make_launch_script(script_dir, script_basename, module_name, path=None):
 class MultiProcessingCmdLineMixin():
     maxDiff = None # Show full tracebacks on subprocess failure
 
-    def setupClass(cls):
-        if cls.start_method not in _concrete_contexts:
-            raise unittest.SkipTest("%r start method not available" %
-                                                          cls.start_method)
+    def setUp(self):
+        if self.start_method not in _concrete_contexts:
+            self.skipTest("%r start method not available" % self.start_method)
 
     def _check_output(self, script_name, exit_code, out, err):
         if verbose > 1:
@@ -277,11 +276,8 @@ class ForkServerCmdLineTest(MultiProcessingCmdLineMixin, unittest.TestCase):
     start_method = 'forkserver'
     main_in_children_source = test_source_main_skipped_in_children
 
-def test_main():
-    support.run_unittest(SpawnCmdLineTest,
-                         ForkCmdLineTest,
-                         ForkServerCmdLineTest)
+def tearDownModule():
     support.reap_children()
 
 if __name__ == '__main__':
-    test_main()
+    unittest.main()