QTestlib: Make QImage comparison more verbose.

QTestlib: Make QImage comparison more verbose.

Introduce a specialization for qCompare(QImage,QImage) that
checks isNull, size and format and outputs verbose messages.

Check isNull, size similarly for QPixmap.

Add an autotest:
- Add test to cmptest and make it a GUI application since
  QImage requires QGuiApplication.
- Make testlib/selftests capable of running X11-GUI applications
  by passing DISPLAY.
- Ignore stderr output for cmptest
- Add test data

Change-Id: I2b29c7822fbeedf2b22c90889739ed7ff859ce92
Reviewed-by: Jason McDonald <jason.mcdonald@nokia.com>