test: fix race in test-net-server-pause-on-connect

A 50ms timeout results in a race condition. Instead, enforce expected
order through callbacks. This has the side effect of speeding up the
test in most situations.

Ref: https://github.com/nodejs/node/pull/4476
PR-URL: https://github.com/nodejs/node/pull/4637


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>