tests: wc,du: add additional --files0-from test cases

tests: wc,du: add additional --files0-from test cases

* tests/wc/wc-files0-from.pl ($limits): New variable.
(@Tests): Prefer the error strings from getlimits over writing them by
hand. Add test cases for --files0-from listing missing files and
duplicate files.
* tests/du/files0-from.pl ($limits): New variable.
(@Tests): Prefer the error strings from getlimits over writing them by
hand. Add test cases for --files0-from listing missing files. Add tests
for --files0-from listing duplicate files with and without the -l option
also in use.