test: prevent flakey test on pi2

Looping rapidly and making new connections causes problems on pi2.
Instead create a new connection when an old connection has already been
made. Running a stress test of 600 times and they all passed.

Fixes: https://github.com/nodejs/node/issues/5302
PR-URL: https://github.com/nodejs/node/pull/5537


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Alexis Campailla <orangemocha@nodejs.org>