test: fix flaky test-child-process-fork-net

Reduce client connections from 10 to 4 in a test that is causing issues
on Raspberry Pi 2 devices in CI.

Fixes: https://github.com/nodejs/node/issues/5122
PR-URL: https://github.com/nodejs/node/pull/6138


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>