test: refactor child-process-fork-net

Split test-child-process-fork-net into
test-child-process-fork-net-server and
test-child-process-fork-net-socket. Rename
test-child-process-fork-net2.js to test-child-process-fork-net.js.

Refs: https://github.com/nodejs/node/pull/21012

PR-URL: https://github.com/nodejs/node/pull/21095


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>