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>

test/cctest/node_test_fixture.h

@@ -41,8 +41,8 @@ struct Argv {
     snprintf(argv_[0], prog_len, "%s", prog);
     snprintf(argv_[0] + prog_len, arg1_len, "%s", arg1);
     snprintf(argv_[0] + prog_len + arg1_len, arg2_len, "%s", arg2);
-    argv_[1] = argv_[0] + prog_len + 1;
+    argv_[1] = argv_[0] + prog_len;
-    argv_[2] = argv_[0] + prog_len + arg1_len + 1;
+    argv_[2] = argv_[0] + prog_len + arg1_len;
   }
 
   ~Argv() {