test: fix test-child-process-fork-regr-gh-2847

It's not guaranteed that the first socket that tries to connect is the
first that succeeds so the rest of assumptions made in the test are not
correct.
Fix it by making sure the second socket does not try to connect until
the first has succeeded.
The IPC channel can already be closed when sending the second socket. It
should be allowed.
Also, don't start sending messages until the worker is online.

Fixes: https://github.com/nodejs/node/issues/8950
PR-URL: https://github.com/nodejs/node/pull/8954


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>