test: fix test-process-exec-argv flakiness

Wait for the `close` event before parsing the child stdout output.

Fixes: https://github.com/nodejs/node/issues/6480
PR-URL: https://github.com/nodejs/node/pull/6575


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>