test: make cluster tests more time tolerant

Port https://github.com/nodejs/node-v0.x-archive/commit/f3f4e282168d243610f8e0241d633ff941c9c260
to master, updating to guard changes for AIX as requested

PR-URL: https://github.com/nodejs/node/pull/2891


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>