[3.10] bpo-45229: Remove test_main in many tests (GH-28405) (GH-28455)

Instead of explicitly enumerate test classes for run_unittest()
use the unittest ability to discover tests. This also makes these
tests discoverable and runnable with unittest.

load_tests() can be used for dynamic generating tests and adding
doctests. setUpModule(), tearDownModule() and addModuleCleanup()
can be used for running code before and after all module tests.
(cherry picked from commit 40348acc18)

Lib/test/test_argparse.py

@@ -12,7 +12,6 @@ import argparse
 
 from io import StringIO
 
-from test import support
 from test.support import os_helper
 from unittest import mock
 class StdIOBuffer(StringIO):
@@ -5397,13 +5396,11 @@ class TestExitOnError(TestCase):
             self.parser.parse_args('--integers a'.split())
 
 
-def test_main():
-    support.run_unittest(__name__)
+def tearDownModule():
     # Remove global references to avoid looking like we have refleaks.
     RFile.seen = {}
     WFile.seen = set()
 
 
-
 if __name__ == '__main__':
-    test_main()
+    unittest.main()