test: fail when child dies in fork-net

Previously when the child dies with errors in this test, the parent
will just hang and timeout, the errors in the child would be
swallowed. This makes it fail so at least there is more information
about why this test fails.

Also removes the unnecessary child.kill() call.

PR-URL: https://github.com/nodejs/node/pull/11684
Ref: https://github.com/nodejs/node/pull/11667


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>