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

首先阅读所引用的 node-mapnik Makefile 中的 make testpack 目标，并将其与此仓库的打包流程进行比较。定义最终软件包应包含的内容，包括 node_modules/node-pre-gyp，并验证该流程能在发布前捕获缺失的依赖项。