[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 40348acc180580371d25f75f46b27048e35f2435)
diff --git a/Lib/test/test_winreg.py b/Lib/test/test_winreg.py
index 487abcd..fedfbe9 100644
--- a/Lib/test/test_winreg.py
+++ b/Lib/test/test_winreg.py
@@ -3,7 +3,6 @@
 
 import os, sys, errno
 import unittest
-from test import support
 from test.support import import_helper
 import threading
 from platform import machine, win32_edition
@@ -490,12 +489,9 @@ def test_exception_numbers(self):
         with self.assertRaises(FileNotFoundError) as ctx:
             QueryValue(HKEY_CLASSES_ROOT, 'some_value_that_does_not_exist')
 
-def test_main():
-    support.run_unittest(LocalWinregTests, RemoteWinregTests,
-                         Win64WinregTests)
 
 if __name__ == "__main__":
     if not REMOTE_NAME:
         print("Remote registry calls can be tested using",
               "'test_winreg.py --remote \\\\machine_name'")
-    test_main()
+    unittest.main()