QA:Testcase gnome3 fallback

Description

This test case tests fallback from composited GNOME Shell to GNOME Panel and Metacity in GNOME 3.

How to test

1. Boot to GNOME 3 on a system which is not capable of the compositing necessary for GNOME Shell: a system whose combination of graphics adapter and driver does not support the necessary compositing features, a virtual machine without the necessary acceleration passthrough, and/or a system whose graphics adapter and driver ought to be capable of displaying the Shell, but which is suffering from a bug

Expected Results

1. Starting GNOME 3 on a system that is not capable of compositing should result in a desktop with GNOME Panel and Metacity instead of GNOME Shell. Aside from visual inspection, confirm that gnome-panel and metacity are running using the command:

   # egrep "(metacity|gnome-panel)" /proc/[0-9]*/cmdline 
   Binary file /proc/6611/cmdline matches
   Binary file /proc/6614/cmdline matches