Improve verification in sampled image tests

Improve verification in sampled image tests

Currently the tests use texelFetch (OpImageFetch) to get sample colors
per texel from a multisampled image and verifies that the color is
either the clear color or the primitive color used. This CL makes
the verification more strict by requiring that at least some texels
have samples of both colors as the image used is multisampled.

Affects:

dEQP-VK.pipeline.multisample.sampled_image.*

Components: Vulkan

VK-GL-CTS issue: 1797

Change-Id: I266538cad5f655ad96eb7221fbdd601b8b055ae8
(cherry picked from commit e0cd1b0113c54ae4e48d8633f29149a4c12cbffe)