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.
Out of curiosity what is this suppose to do?
Please tell me you're not making the root owned by the `worker` users :)
If it's supposed to operate on the files added to `/home/worker/`, ie. current working directory, I would suggest something like `chown -R worker:worker /home/worker/` just be less confusing... (note I didn't test the command, so not sure if `-R` will do the trick :)