Eg. Current graphics_VTSwitch tests takes 6 screenshots, each screenshots takes about 12MB. So, at least 72 MB is used for storing the screenshots. We don't like these images to be throttled (the current limit is 20MB) and DDOS the lab by storing lots of images. 

So, let's remove the images/screenshots when the test is passed.
Currently, the tests to be considered are
graphics_Sanity
graphics_VTSwitch