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
I think we need to set testAgentRuntime at some point, so that https://github.com/gaye/gaia/blob/274a058ca889ac5fff3c4ae73abe4796d0fc7ded/apps/calendar/test/unit/setup.js#L78 works