test: improve coverage of ModuleMap.js

PR-URL: https://github.com/nodejs/node/pull/15924


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>