test: increase the platform timeout for AIX

There have been failures on AIX due to the slower
default loopback performance. So far I've resisted
updating the global timeout but seeing another
new failure in a newly added test I now think the
right thing is to just extend the platform
timeout for AIX. This commit does that.

PR-URL: https://github.com/nodejs/node/pull/6342


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>