child_process: internal cleanup and input validation improvements

This PR is split up into three commits:

1. Cleanup of the internal child_process source file. Removes some self = this assignments and replaces some chained array iteration functions with one simple for loop.
2. Cleanup of the ChildProcess test in preparation for the third commit.
3. Add input validation. ChildProcess wasn't always an official public API. When we made it official, we didn't add any input validation. It was pretty easy to pass bad input to it. Got the idea from (Refs) #12177 (closed).

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

child_process, test