tests/shader-runner: Query for SM6 support instead of checking macro.
2/2 avoids skipping the SM6 tests on a Windows build, where the support is present but the VKD3D_SHADER_UNSUPPORTED_DXIL macro is not defined; a mistake I introduced with !1109 (merged).
Thanks @giomasce for pointing this out and suggesting querying the backend instead, this is also done for the GLSL backend in 1/2.