test: mitigate flaky test-http-agent

Reduce number of clients from 100 to 16 as Raspberry Pi in CI starts to
exhibit flakiness around 22 or so clients.

Fixes: https://github.com/nodejs/node/issues/5184
PR-URL: https://github.com/nodejs/node/pull/5346


Reviewed-By: James M Snell <jasnell@gmail.com>