Add 8-bit blend_operation_advanced tests

Add 8-bit blend_operation_advanced tests

This change adds 8-bit versions of the VK_EXT_blend_operation_advanced
tests, adjusting error threshold accordingly.

This change also adds a new variant of the image comparison function,
fixing the original tests' functionality to skip some pixels from
comparison.

Affects:
dEQP-VK.pipeline.blend_operation_advanced.*

New tests:
dEQP-VK.pipeline.blend_operation_advanced.*_r8g8b8a8_unorm

Components: Vulkan
VK-GL-CTS issue: 3233

Change-Id: I155b825d7210730bf8cf4248346e81411c2b6cb3