[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_ftplib.py b/Lib/test/test_ftplib.py
index 3492ba4..56e3d8a 100644
--- a/Lib/test/test_ftplib.py
+++ b/Lib/test/test_ftplib.py
@@ -10,6 +10,7 @@
 import os
 import threading
 import time
+import unittest
 try:
     import ssl
 except ImportError:
@@ -1144,18 +1145,10 @@ def test__all__(self):
         support.check__all__(self, ftplib, not_exported=not_exported)
 
 
-def test_main():
-    tests = [TestFTPClass, TestTimeouts,
-             TestIPv6Environment,
-             TestTLS_FTPClassMixin, TestTLS_FTPClass,
-             MiscTestCase]
-
+def setUpModule():
     thread_info = threading_helper.threading_setup()
-    try:
-        support.run_unittest(*tests)
-    finally:
-        threading_helper.threading_cleanup(*thread_info)
+    unittest.addModuleCleanup(threading_helper.threading_cleanup, *thread_info)
 
 
 if __name__ == '__main__':
-    test_main()
+    unittest.main()