Use highp by default in ShaderLibrary-based tests

Use highp by default in ShaderLibrary-based tests

This resolves numerous RelaxedPrecision mismatches

Affects:

dEQP-VK.glsl.arrays.*
dEQP-VK.glsl.conditionals.*
dEQP-VK.glsl.constants.*
dEQP-VK.glsl.conversions.*
dEQP-VK.glsl.functions.*
dEQP-VK.glsl.linkage.*
dEQP-VK.glsl.scoping.*
dEQP-VK.glsl.swizzles.*

See #518

Change-Id: I7b79f10c6dbdd5ae8da31359b680ac52ce2ad28c