test: reduce timeouts in test-net-keepalive

Previously 1000-1200ms, they're now (platform dependent) 50-100ms.

Improves test run time on my machine from 0m1.335s to 0m0.236s.

PR-URL: https://github.com/nodejs/node/pull/2429


Reviewed-By: Rich Trott <rtrott@gmail.com>