Add more unit tests for qsgcanvasitem and fix unstable tests

Change-Id:I5fc11a5874d55ad423dc1fb9c3e1b75a38003465
Reviewed-on: http://codereview.qt-project.org/5962


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Charles Yin <charles.yin@nokia.com>