Fix image type in Uniform texel buffer tests

Fix image type in Uniform texel buffer tests

Form the issue:
The Shader Resource and Storage Class Correspondence table
in the Descriptor Set Interface section of the Vulkan 1.1
specification clearly states that a descriptor resource
which is a uniform texel buffer should correspond to
an OpTypeImage(Dim=Buffer,Sampled=1) type.

The current tests in the binding_model category
generate OpTypeSampledImage for uniform texel buffer
resources instead, which I believe is incorrect.

This can be fixed by changing the GLSL declaration from
samplerBuffer to textureBuffer

Components: Vulkan

VK-GL-CTS public issue: 139

Affects:
dEQP-VK.binding_model.*uniform_texel_buffer.*

Change-Id: Ie1beb05cac69a6cf1220bbd47c2d7876eed7ff26