Bug 1002545 - Downloading firefox test package instead of xulrunner sdk

The current xulrunner package, when gaia is included in b2g builds, is not sufficient to run mochitests.  This downloads firefox and the accompanying tests package in order to get xpcshell and a compatible version of ssltunnel.
we shoud keep using `b2g_sdk-<VERSION>` since it's annoying if we switch between different branches then trigger re-download b2g/xulrunner and we fixed it on [bug 906316](https://bugzilla.mozilla.org/show_bug.cgi?id=1002545)