[HLSL][RootSignature] Add test to demonstrate `offset = append` behaviour

[inbelic]

From implementing the spec, there were questions about if we need to/can compute the manual offsets before pass down an offset value to the construction of a root signature in the d3d api.

AC:

• add test to demonstrate how offset= append should behave if passed in the magic value D3D12_DESCRIPTOR_RANGE_OFFSET_APPEND