bpo-40275: Adding threading_helper submodule in test.support (GH-20263)

diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 81e5f70..ad82e30 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -3,6 +3,7 @@
 """
 
 import test.support
+from test.support import threading_helper
 from test.support import verbose, import_module, cpython_only
 from test.support.script_helper import assert_python_ok, assert_python_failure
 
@@ -75,10 +76,10 @@
 
 class BaseTestCase(unittest.TestCase):
     def setUp(self):
-        self._threads = test.support.threading_setup()
+        self._threads = threading_helper.threading_setup()
 
     def tearDown(self):
-        test.support.threading_cleanup(*self._threads)
+        threading_helper.threading_cleanup(*self._threads)
         test.support.reap_children()
 
 
@@ -130,7 +131,7 @@
             done.set()
         done = threading.Event()
         ident = []
-        with support.wait_threads_exit():
+        with threading_helper.wait_threads_exit():
             tid = _thread.start_new_thread(f, ())
             done.wait()
             self.assertEqual(ident[0], tid)
@@ -171,7 +172,7 @@
 
         mutex = threading.Lock()
         mutex.acquire()
-        with support.wait_threads_exit():
+        with threading_helper.wait_threads_exit():
             tid = _thread.start_new_thread(f, (mutex,))
             # Wait for the thread to finish.
             mutex.acquire()