test: mitigate flaky test-https-agent

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

PR-URL: https://github.com/nodejs/node/pull/5939
Fixes: https://github.com/nodejs/node/issues/5938
Refs: https://github.com/nodejs/node/issues/5184


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>