test: fix flaky test-preload

Checklist

• tests and code linting passes
• the commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

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

Fixes: https://github.com/nodejs/node/issues/6722

/cc @Fishrock123 @mhdawson h/t @santigimeno