tests: Add test for NULL-blit warnings
tests: Add test for NULL-blit warnings

Extended an image test to look for warnings upon calling vkCmdBlitImage
with NULL blit extents.

Change-Id: I5a8f34c2ac8b9a1f074311a16e8635cf34c1e2bc

elongbug/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,elongbug/Vulkan-LoaderAndValidationLayers,elongbug/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,elongbug/Vulkan-LoaderAndValidationLayers