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

The test is still failing sometimes because when trying to establish the
second connection, the server is already closed. Bring back the code
that handled this case and was removed in the last refactoring of the
test. Also ignore the errors that might happen when sending the second
handle to the worker because it may already have exited.

PR-URL: https://github.com/nodejs/node/pull/5422


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