Add make testpack

[GretaCB]

Per convo with @flippmoke, I learned about make testpack which is used in node-mapnik to ensure the final package looks good to officially publish and to avoid dependency issues later in the chain.

Would be great for skel to have a workflow in place for ensuring the final package contains everything we'd expect, for example node_modules/node-pre-gyp/

cc @springmeyer @millzpaugh @mapsam