test: fix flaky cluster-net-send

Before this commit, it was possible on Windows for the server's
'connection' handler to be called *after* the client socket's
'connect' handler. This caused the 'message' event to be missed
and the test would never end (timing out in CI). This problem
was more easily reproducible on a low resource (slow CPU)
Windows (2012r2) installation.

This commit waits until both handlers have been called before
sending the handle to the master process.

Fixes: https://github.com/nodejs/node/issues/3957
PR-URL: https://github.com/nodejs/node/pull/4444


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