tests/shader_runner: Add support for testing explicit descriptor mapping.

When no descriptor mapping is specified, the backend will just
build the usual default mapping. Otherwise the explicit mapping
is used.

Once all backends support the explicit mapping, we'll be able to
handle generating the default mapping in the shader runner core
rather than having each backend implement its own algorithm.

So far only the d3d12 backend supports explicit descriptor
mapping.

tests/hlsl/descriptors.shader_test

@@ -0,0 +1,98 @@
+[require]
+shader model >= 5.0
+descriptors
+
+[srv 0]
+format r32-float
+size (2d, 1, 1)
+
+1.0
+
+[srv 1]
+format r32-float
+size (2d, 1, 1)
+
+2.0
+
+[srv 2]
+format r32-float
+size (2d, 1, 1)
+
+3.0
+
+[pixel shader]
+Texture2D<float> t : register(t1);
+
+float4 main() : SV_Target
+{
+    return t.Load(uint3(0, 0, 0));
+}
+
+[test]
+draw quad
+probe (0, 0) f32(2.0, 2.0, 2.0, 2.0)
+
+[descriptors]
+t[1:1], space 0, srv 2
+
+[test]
+draw quad
+probe (0, 0) f32(3.0, 3.0, 3.0, 3.0)
+
+[descriptors]
+t[1:2], space 0, srv 0
+
+[test]
+draw quad
+probe (0, 0) f32(1.0, 1.0, 1.0, 1.0)
+
+[require]
+shader model >= 5.0
+descriptors
+format r32-float uav-load
+
+[uav 1]
+format r32-float
+size (2d, 1, 1)
+
+1.0
+
+[uav 2]
+format r32-float
+size (2d, 1, 1)
+
+2.0
+
+[uav 3]
+format r32-float
+size (2d, 1, 1)
+
+3.0
+
+[pixel shader]
+RWTexture2D<float> u : register(u2);
+
+float4 main() : SV_Target
+{
+    return u[uint2(0, 0)];
+}
+
+[descriptors]
+
+[test]
+todo(msl) draw quad
+probe (0, 0) f32(2.0, 2.0, 2.0, 2.0)
+
+[descriptors]
+u[2:2], space 0, uav 3
+
+[test]
+draw quad
+probe (0, 0) f32(3.0, 3.0, 3.0, 3.0)
+
+[descriptors]
+u[2:3], space 0, uav 1
+
+[test]
+draw quad
+probe (0, 0) f32(1.0, 1.0, 1.0, 1.0)