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_mailcap.py

@@ -213,9 +213,5 @@ class FindmatchTest(unittest.TestCase):
             self.assertEqual(mailcap.findmatch(*c[0], **c[1]), c[2])
 
 
-def test_main():
-    test.support.run_unittest(HelperFunctionTest, GetcapsTest, FindmatchTest)
-
-
 if __name__ == '__main__':
-    test_main()
+    unittest.main()