Test omitting blend state with unused attachments

Test omitting blend state with unused attachments

This test creates a render pass with a single color attachment and two
subpasses. The first subpass uses the color attachment and creates a
graphics pipeline with a valid color blend state. The second subpass
sets the color attachment to VK_ATTACHMENT_UNUSED and creates a
graphics pipeline without a color blend state.

A new helper function was added for creating a graphics pipeline.

New tests:

dEQP-VK.renderpass.*.simple.color_unused_omit_blend_state

Components: Vulkan, Framework

VK-GL-CTS issue: 1115

Change-Id: I1d95dab7db5727ded22fe84aca5959110949f581