[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_iter.py b/Lib/test/test_iter.py
index 20cb9c0..554f602 100644
--- a/Lib/test/test_iter.py
+++ b/Lib/test/test_iter.py
@@ -2,7 +2,7 @@
 
 import sys
 import unittest
-from test.support import run_unittest, cpython_only
+from test.support import cpython_only
 from test.support.os_helper import TESTFN, unlink
 from test.support import check_free_after_iterating, ALWAYS_EQ, NEVER_EQ
 import pickle
@@ -1037,9 +1037,5 @@ def test_error_iter(self):
         self.assertRaises(ZeroDivisionError, iter, BadIterableClass())
 
 
-def test_main():
-    run_unittest(TestCase)
-
-
 if __name__ == "__main__":
-    test_main()
+    unittest.main()