test: add coverage for utf8CheckIncomplete()

This commit adds code coverage to utf8CheckIncomplete(), when the
lastNeed is greater than buffer length.

PR-URL: https://github.com/nodejs/node/pull/11419


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>