test,child_process: add tests for signalCode value

Prior to this change, none of the child_process tests checked the
signalCode property for a value other than null. Add a check to an
existing test.

PR-URL: https://github.com/nodejs/node/pull/35327


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>