test: fix flaky test-stdout-close-catch

Make sure that `catch-stdout-error` has written data before the
destination process exits.

Fixes: https://github.com/nodejs/node/issues/6791
PR-URL: https://github.com/nodejs/node/pull/6808


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>