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.
since we handled different OSes for `XPCSHELLSDK`, seems it's not necessary to guess xpcshellsdk here.