bpo-31904: fix test_doctest.py failures for VxWorks (GH-23419)

Fix test_doctest.py failures for VxWorks by avoiding exact error message checks. (better for everyone all around)
diff --git a/Lib/test/test_doctest.py b/Lib/test/test_doctest.py
index bff20f9..6a5013f 100644
--- a/Lib/test/test_doctest.py
+++ b/Lib/test/test_doctest.py
@@ -3039,10 +3039,11 @@ def test_CLI(): r"""
     ...         '-m', 'doctest', 'nosuchfile')
     >>> rc, out
     (1, b'')
+    >>> # The exact error message changes depending on the platform.
     >>> print(normalize(err))                    # doctest: +ELLIPSIS
     Traceback (most recent call last):
       ...
-    FileNotFoundError: [Errno ...] No such file or directory: 'nosuchfile'
+    FileNotFoundError: [Errno ...] ...nosuchfile...
 
 Invalid doctest option: