test: extend timeout in Debug mode

Debug mode slows execution speed. There is work afoot to enable Debug
mode runs on the continuous integration infrastructure for the project.
Some tests are timing out, such as test-net-GH-5504.js.

This change doubles the timeout returned from `common.platformTimeout()`
when running in Debug mode. It also removes an unused variable from the
aforementioned test-net-GH-5504.js.

PR-URL: https://github.com/nodejs/node/pull/4431


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>