Test interaction of image_view_min_lod, robustness2 and gather

Test interaction of image_view_min_lod, robustness2 and gather

This commits adds a new set of tests that verify texture gather
operations interact correctly with robustness2 and image_view_min_lod.
Specifically, texture gathering always retrieves pixel values from the
base level, and image_view_min_lod may not make that level available on
the sampled texture. With robustness2, reading from those texels should
result in zeros.

New tests:
dEQP-VK.texture.mipmap.min_lod_gather.*

Components: Vulkan
VK-GL-CTS issue: 3808

Change-Id: Id6e5fe445e45e75bb419bebe0da59ea22cf452d9