(PUP-2350) Fixup Windows tests for removal of non-string file mode support

This commit updates several Windows-only spec tests so that they
adhere to the recent removal of support for non-string file modes.
Merge pull request #3156 from whopper/pup-2350/file-mode-test-fixup

(PUP-2350) Fixup Windows tests for removal of non-string file mode support