test: mark test-child-process-exit-code flaky

Refs: # https://github.com/nodejs/node/issues/25033

PR-URL: https://github.com/nodejs/node/pull/25050


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>