Test reading compute shader builtin vars by component

Test reading compute shader builtin vars by component

- Adds a test of reading compute shader builtin variables by component.
  These get test case name suffix of "_component".
- Previous behaviour, reading the whole builtin variable, is preserved
  in cases but without an extra suffix.
- WorkgroupSize builtin is made specializable to prevent Glslang
  from constant folding its value.

Affects:
Components: Vulkan

dEQP-VK.compute.builtin_var.*

VK-GL-CTS issue: 694

Change-Id: Ia783babf4ba5e24fc58bc9d19594bb7d921de7c9