test,doc: make module name match gyp target name

Currently the nm_modname does not match the file name of the resulting
module. In fact, the nm_modname is pretty arbitrary. This seeks to
introduce some consistency into the nm_modname property by having the
name of the module appear in exactly one place: the "target_name"
property of the gyp target that builds the module.

PR-URL: https://github.com/nodejs/node/pull/15209


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>