test: address flaky test-http-client-timeout-event

Use common.platformTimeout() to make test more reliable on Raspberry Pi.

Fixes: https://github.com/nodejs/node/issues/2555
PR-URL: https://github.com/nodejs/node/pull/3968



Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>