Test ViewportIndex SPIR-V builtin in frag shaders

Test ViewportIndex SPIR-V builtin in frag shaders

Add new tests checking the ViewportIndex SPIR-V builtin, when used as
gl_ViewportIndex from GLSL, works properly in fragment shaders.

New tests:
dEQP-VK.draw.shader_viewport_index.fragment_shader_*

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

Components: Vulkan
VK-GL-CTS issue: 2262

Change-Id: Ib66326bab17177d59bbc0802e1cdc7534d825a93