[3.10] bpo-31904: Correct error string in test_file_not_exists() for VxWorks (GH-25965) (GH-25983)

The error string on VxWorks is "no such file or directory" for FileNotFoundError. That is, the 1st letter of the error string has lower case.
(cherry picked from commit b063b02eabf91bfd4edc0f3fde7ce8f0ebb392c4)


Co-authored-by: pxinwr <peixing.xin@windriver.com>

Automerge-Triggered-By: GH:gpshead