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
maybe it's a good idea to bring a "defer" algorithm like we have in https://github.com/mozilla-b2g/gaia/blob/12d634c63ec8a41553a9f150c38260c992dc0020/apps/sms/js/utils.js#L634-L643, this would keep us the same indentation