Improve hardening-tests of child_process.fork() once Node.js version is upgraded

[watson]

Depends on #47968

In #55697, new tests are added that tests the hardening of the child_process module. Some tests of the fork function have been omitted as the version of Node.js that Kibana is currently running contains a bug which makes some corner-case testing of fork impossible (see TODO in #55697).

Once we upgrade to a newer version of Node.js, these tests should be added.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)