Accept per-sample shading in multiple interpolation tests

Accept per-sample shading in multiple interpolation tests

When using the sample decoration in one of the fragment shader input
variables, some implementations may switch to per-sample shading mode,
producing results which are different from the per-fragment shading
results but which are allowed by the spec.

This commit enables sample shading with rate 1.0 as an alternative valid
result for multiple interpolation tests in implementations supporting
the sampleRateShading feature.

Affected tests:
dEQP-VK.draw.multiple_interpolation.*

Components: Vulkan
VK-GL-CTS issue: 2993

Change-Id: Id5d33df38b8c7179a0686b615578871aea8d8f22