test: add coverage for child_process bounds check

Make sure that monkey-patching process.execArgv doesn't cause
child_process to incorrectly munge execArgv in fork().

This basically is adding coverage for an `index > 0` check (see Refs).
Previously, that condition was never false in any of the tests.

PR-URL: https://github.com/nodejs/node/pull/11800


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Refs: https://github.com/nodejs/node/blob/c67207731f16a78f6cae90e49c53b10728241ecf/lib/child_process.js#L76