bpo-40275: Use new test.support helper submodules in tests (GH-21219)

diff --git a/Lib/test/test_poll.py b/Lib/test/test_poll.py
index a14c69a..de62350 100644
--- a/Lib/test/test_poll.py
+++ b/Lib/test/test_poll.py
@@ -7,8 +7,10 @@
 import threading
 import time
 import unittest
-from test.support import TESTFN, run_unittest, cpython_only
+from test.support import run_unittest, cpython_only
 from test.support import threading_helper
+from test.support.os_helper import TESTFN
+
 
 try:
     select.poll