test: fix test-net-persistent-keepalive for AIX

Fixed an intermittent issue on AIX where the 600ms timeout was reached
before the 'connection' event was fired. This resulted in a failure as
serverConnection would be undefined and the assert.equal would throw an
error. Changed the flow of the test so that the timeout is only set
after a connection has been made.

PR-URL: https://github.com/nodejs/node/pull/3646


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>