Bug 1027249: Add make targets for b2g-emulator tests

Here are the make targets and associated support files I've been using to run b2g-emulator tests. I'm not sure these are practical / useful for production, but they capture everything that is needed to run all the different test flavors.

(A minor change also restricts b2g-desktop tests to the sanity reftests -- the only ones that pass consistently, and the only ones run on tbpl.)