doc: recommend using port 0 instead of common.PORT

In the 'writing_tests' guide it is recommended to use common.PORT
instead of an arbitrary value, but the recommendation is to use
port 0 instead and the docs should reflect that.

PR-URL: https://github.com/nodejs/node/pull/8694


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>