[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_distutils.py b/Lib/test/test_distutils.py
index 90bfc70..4b40af0 100644
--- a/Lib/test/test_distutils.py
+++ b/Lib/test/test_distutils.py
@@ -15,16 +15,14 @@
     import distutils.tests
 
 
-def test_main():
-    # used by regrtest
-    support.run_unittest(distutils.tests.test_suite())
-    support.reap_children()
-
-
 def load_tests(*_):
     # used by unittest
     return distutils.tests.test_suite()
 
 
+def tearDownModule():
+    support.reap_children()
+
+
 if __name__ == "__main__":
-    test_main()
+    unittest.main()