test: only include http module once

A few tests in test/gc include the http module twice. Remove duplicate
require().

PR-URL: https://github.com/nodejs/node/pull/4606


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>