Unify getShaderStageName helper

Unify getShaderStageName helper

Clean out helper functions used to create shader stage
and precision names.

Note: affects only the test case name generation.

Affects:

dEQP-VK.ycbcr.format.*
dEQP-VK.glsl.builtin.function.*
dEQP-VK.protected_memory.ssbo.ssbo_atomic.*

Components: Vulkan, Framework, OpenGL

VK-GL-CTS issue: 1736

Change-Id: Id326c3c241ecf1a60c0dda3ee8c31b155114fc3c