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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
test