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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines