gpu_test hangs if it can't find glxgears windows using wmctrl

Steps to reproduce:

- Remove wmctrl and glxgears
- CHECKBOX_SHARE=/usr/share/checkbox checkbox-old/scripts/gpu_test

Expected result:
- Test should exit gracefully

Actual result:
- only html video window launches
- Trace as seen below
- Test never exits, stalling the test run.

This happens sporadically on actual runs, even if glxgears is installed the script seems unable to find the window and throws this trace.

Traceback (most recent call last):
  File "checkbox-old/scripts/gpu_test", line 220, in <module>
    sys.exit(main())
  File "checkbox-old/scripts/gpu_test", line 171, in main
    GlxRotate.append(RotateGlxThread(GlxWindows[i].id, i + 1))
AttributeError: 'GlxThread' object has no attribute 'id'

I think the actual meaning of "id" is "window id", as this is thrown when wmctrl can't find any glxgears window.