test: refactor test-http-agent-timeout-option

Fix flakyness caused by usage of a non-routable IP address.

Refs: https://github.com/nodejs/node/pull/25488#issuecomment-459385146

PR-URL: https://github.com/nodejs/node/pull/25854


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>