[clang] [HLSL][TEST] Fix root signature driver test on WSL (PR #161566)

[Sarah Spall via cfe-commits]

spall wrote:

> Why is the extension different under WSL? This change might be reasonable, but I find that difference quite surprising and wonder if this is masking another bug. Can we look into the answer to that before making this change please?

from googling it seems that the dxo file type is not supported on linux? But before looking at this PR I'd never heard of this file type, so its possible I'm not reading about the correct one.

https://github.com/llvm/llvm-project/pull/161566