build: make ci test addons in test/addons

Make `make test-addons` part of the `make test-ci` target.

Use order-only prerequisites to make generating and building the add-ons
concurrency-safe when $JOBS > 1 and fudge the dependency on $(NODE_EXE)
so that add-ons are only rebuilt when needed instead of all the time.

PR-URL: https://github.com/nodejs/node/pull/2428


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rod Vagg <rod@vagg.org>