tests: fold: avoid false failures on Solaris 10

tests: fold: avoid false failures on Solaris 10

* tests/fold/fold-characters.sh: Ensure we have independent verification
of the width of characters before testing based on those widths.
* tests/fold/fold-zero-width.sh: Likewise.
* tests/fold/fold.pl: Only compare the exit status,
as the error message can be translated.