Merge branch '270-spec-constant-tests' into 'vulkan-cts-1.0-dev'

Merge branch '270-spec-constant-tests' into 'vulkan-cts-1.0-dev'

Add specialization constants tests

Adds the spec. constant. tests for graphics and compute pipelines.

Includes:
- GLSL-based shaders
- Basic, vector, matrix, struct, array constants
- Work group size specialization
- Built-in constant specialization
- Spec. const as array size
- Test spec document.

Excludes:
- Spec. constant expressions (OpSpecConstantOp), as they are tested
  extensively in assembly tests.

Fixes issue #270 and #204

See merge request !413