bpo-41069: Make TESTFN and the CWD for tests containing non-ascii characters. (GH-21035)

diff --git a/Lib/test/test_posixpath.py b/Lib/test/test_posixpath.py
index 4d3d897..18819a5 100644
--- a/Lib/test/test_posixpath.py
+++ b/Lib/test/test_posixpath.py
@@ -627,7 +627,7 @@
     path = posixpath
 
     def setUp(self):
-        self.file_name = support.TESTFN.lower()
+        self.file_name = support.TESTFN
         self.file_path = FakePath(support.TESTFN)
         self.addCleanup(support.unlink, self.file_name)
         with open(self.file_name, 'xb', 0) as file: