bpo-45229: Remove test_main in many tests (GH-28405)

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.

Lib/test/test_unicode_file_functions.py

@@ -5,7 +5,6 @@
 import unittest
 import warnings
 from unicodedata import normalize
-from test import support
 from test.support import os_helper
 
 
@@ -185,15 +184,5 @@ class UnicodeNFKDFileTests(UnicodeFileTests):
     normal_form = 'NFKD'
 
 
-def test_main():
-    support.run_unittest(
-        UnicodeFileTests,
-        UnicodeNFCFileTests,
-        UnicodeNFDFileTests,
-        UnicodeNFKCFileTests,
-        UnicodeNFKDFileTests,
-    )
-
-
 if __name__ == "__main__":
-    test_main()
+    unittest.main()