Single color buffer in multiple interpolation tests

Single color buffer in multiple interpolation tests

The multiple interpolation tests were requiring 5 color attachments to
run the main fragment shader, when the spec requires implementations to
support at least 4.

Instead of checking the number of supported color attachments and
marking the tests as not supported in those cases, this commit changes
the main fragment shader to use only one color attachment and selects
one of the multiple interpolations using push constants.

The tests run a bit slower because they use multiple pipelines and
render passes in sequence instead of a single one, but they can be run
on any implementation.

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

Components: Vulkan
VK-GL-CTS issue: 2986

Change-Id: I14f0aacd8e134bbcbf027f46ed6c18076bae09f5