Add make testpack

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

先閱讀所參照的 node-mapnik Makefile 中的 make testpack 目標，並將其與此儲存庫的封裝流程進行比較。定義最終套件應包含的內容，包括 node_modules/node-pre-gyp，並驗證該流程能在發布前擷取遺漏的相依性。