[3.12] gh-89392: Remove test_main() in test_netrc (GH-108860) (#108872)

[3.12] gh-89392: Remove test_main() in test_netrc (GH-108860).
(cherry picked from commit 76f3c04)

Co-authored-by: T. Wouters <thomas@python.org>

Author: serhiy-storchaka

Diff for: Lib/test/test_netrc.py

@@ -1,5 +1,5 @@
 import netrc, os, unittest, sys, textwrap
-from test.support import os_helper, run_unittest
+from test.support import os_helper
 
 try:
     import pwd
@@ -308,8 +308,6 @@ def test_security(self):
             self.assertEqual(nrc.hosts['foo.domain.com'],
                              ('anonymous', '', 'pass'))
 
-def test_main():
-    return run_unittest(NetrcTestCase)
 
 if __name__ == "__main__":
-    test_main()
+    unittest.main()