Add tests for VK_AMD_texture_gather_bias_lod

Add tests for VK_AMD_texture_gather_bias_lod

This commit adds new texture gather test variants to exercise functions
from VK_AMD_texture_gather_bias_lod. Both the optional bias argument and
the explicit lod parameters are tested.

New tests:
dEQP-VK.glsl.texture_gather.*amd*

Affected tests:
dEQP-VK.glsl.texture_gather.*

Components: Vulkan
VK-GL-CTS issue: 2353

Change-Id: If99e4f123676040fecf0cb95c0af8c8df27441d8