test: fix test-net-persistent-keepalive for AIX
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 <241bf52ebd97cc4ec83d57b04d1f9e9053df032e@ca.ibm.com>
Reviewed-By: James M Snell <2d64a127ffc94dbe6295928a16bedf8b26814159@gmail.com>

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