make npm test pass

run `npm test` will be an error.

It's enough to have `"test": "mocha"` here: `npm test` itself can take an installed module into account.

Having `"test": "mocha"` also has an added benefit of supporting a globally-installed Mocha (i.e. `npm install mocha -g`) as well as locally-installed (i.e. `npm install mocha` without `-g`).