gh-96189: Fix test_invalid_utf8 on a number of build bots (GH-96190)

The clearing of the temporary directory is not working on some platforms and
leaving behind files.

This has been updated to use the pattern in test_cmd_line.py [1] using the
special TESTFN rather than a test directory.

[1] https://github.com/python/cpython/blob/main/Lib/test/test_cmd_line.py#L559

Lib/test/test_source_encoding.py

@@ -239,8 +239,8 @@ def test_invalid_utf8(self):
         # it's an otherwise valid Python source file.
         template = b'"%s"\n'
 
-        with tempfile.TemporaryDirectory() as tmpd:
-            fn = os.path.join(tmpd, 'test.py')
+        fn = TESTFN
+        self.addCleanup(unlink, fn)
 
         def check(content):
             with open(fn, 'wb') as fp: