bpo-40275: Use new test.support helper submodules in tests (GH-21764)

diff --git a/Lib/test/test_startfile.py b/Lib/test/test_startfile.py
index 1a26a80..589ffa2 100644
--- a/Lib/test/test_startfile.py
+++ b/Lib/test/test_startfile.py
@@ -9,6 +9,7 @@
 
 import unittest
 from test import support
+from test.support import os_helper
 import os
 import platform
 import sys
@@ -27,7 +28,7 @@
         # we're not about to delete. If we're running under -j, that
         # means the test harness provided directory isn't a safe option.
         # See http://bugs.python.org/issue15526 for more details
-        with support.change_cwd(path.dirname(sys.executable)):
+        with os_helper.change_cwd(path.dirname(sys.executable)):
             empty = path.join(path.dirname(__file__), "empty.vbs")
             startfile(empty)
             startfile(empty, "open")