Add test for dynamically indexing output variable components

Add test for dynamically indexing output variable components

Added test where output variable component is dynamically selected
using input data. Graphics test framework was modified to support
code inside main function as the output variables are not visible
inside a test function. Fixed support for vec4 output variables in
the same framework. Previously it was resulting in a code that wasn't
accepted by the validator.

Affects:

dEQP-VK.spirv_assembly.*

Components: Vulkan

VK-GL-CTS issue: 1122

Change-Id: I78f149b0f709e1d11dd977cc432fec0e5f971bed