test: rename target to exports for consistency

The doc/api/addons.md document contains examples of Addon
Initialization functions that take a parameter named exports.
This also matches the name used in node.cc when calling:
mp->nm_register_func(exports, module, mp->nm_priv);

Currently, a number of the tests name this same parameter target. This
commit renames target to exports for consistency.

PR-URL: https://github.com/nodejs/node/pull/9135


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>