Test interpolation functions with single components

Test interpolation functions with single components

This commits adds new variants to some multisample interpolation tests
to check single-component versions of interpolation functions, when the
component is accessed both as a constant and a non-constant expression
in the form of shader push constants.

New and affected tests:
dEQP-VK.pipeline.multisample_interpolation.*

Components: Vulkan
VK-GL-CTS issue: 2259

Change-Id: Ic1e825c850ac085a6f2d9ee5fa572918c144805b