test: really run addon tests on `make test`

Fix a `s/addon/addons/` typo in the Makefile.

PR-URL: https://github.com/nodejs/node/pull/7542


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>