tests do not always pass on older perls when piping to stdin, despite IO::Pty supposedly reopening a tty for us

e.g. https://s3.amazonaws.com/archive.travis-ci.org/jobs/40737197/log.txt
Dist-Zilla-Plugin-PromptIfStale-0.030

          - make prompt tests unfailable on perls older than 5.16, to work around an
            apparent issue with IO::Pty