Test zero-size unused specialization constants

Test zero-size unused specialization constants

According to the spec for VkSpecializationMapEntry

> If a constantID value is not a specialization constant ID used in the
> shader, that map entry does not affect the behavior of the pipeline.

So it should be legal to use VkSpecializationMapEntry structures
indicating size zero for a given constant as long as the constant is not
used in the shaders.

This commits adds new test variants that verify implementations handle
such cases.

New tests:
dEQP-VK.pipeline.spec_constant.*.basic.*unused_*

Affected tests:
dEQP-VK.pipeline.spec_constant.*

Components: Vulkan
VK-GL-CTS issue: 3011

Change-Id: I2a8439368722e6caaa828fd91e061cfd5e2be61d