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 comment
in this commit). Previously, that condition was never false in any of
the tests.

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)

test child_process