Issue #21741: Update 147 test modules to use test discovery.

I have compared output between pre- and post-patch runs of these tests
to make sure there's nothing missing and nothing broken, on both
Windows and Linux.  The only differences I found were actually tests
that were previously *not* run.

Lib/test/test_grp.py

@@ -92,8 +92,5 @@ class GroupDatabaseTestCase(unittest.TestCase):
 
         self.assertRaises(KeyError, grp.getgrgid, fakegid)
 
-def test_main():
-    support.run_unittest(GroupDatabaseTestCase)
-
 if __name__ == "__main__":
-    test_main()
+    unittest.main()