Bug 1068236 - Support 3p module loaders as first-class citizens in test-agent r=julienw

A @lightsofapollo patch to allow test writers to overwrite test-agent's test loader
wondering if `return Promise.all(helpers.map(box.require.bind(box)))` is not as good, if not better? (if you prefer an arrow function to the binding, it's good to me too)