test: fix truncation of argv

Currently argv_[1] and argv_[2] are getting truncated by one character
because of an incorrect addition of one to account for the null
character. I only noticed this when working on #12087, but that fix
will probably not get included in favor of a JavaScript test so I'm
adding this separate commit for it.

Refs: https://github.com/nodejs/node/pull/12087
PR-URL: https://github.com/nodejs/node/pull/12110


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>