test: fix flaky test-preload

Use `close` event rather than `exit` event to make sure all output has
been received before checking assertions.

PR-URL: https://github.com/nodejs/node/pull/6728
Fixes: https://github.com/nodejs/node/issues/6722


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>