Bug 1027249: Add support for b2g emulator tests

These are the changes I needed to run b2g emulator tests: Install various extra packages and run Xvfb in the background instead of xvfb-run. I also opted to remove test_setup.sh and capture those steps in the tester Dockerfile instead.
These are referencing files in /home/worker: the files added by ADD are all owned by root otherwise. I can make that more explicit.