Added tests for rasterizer discard

Added tests for rasterizer discard

New tests were added to verify correct functionality when setting
rasterizerDiscardEnable to VK_TRUE in rasterizationStateCreateInfo.
Tests verify that primitives are discarded before rasterization by
comparing the rendered image to a blank reference image and ensuring
that no fragment shader invocations have occured.

New tests:

dEQP-VK.rasterization.discard.*

Components: Vulkan

VK-GL-CTS issue: 1149

VK-GL-CTS public issue: 100

Change-Id: Ided170a2831efaac7e393ce8cafb93269180b3d0