[llvm] [SPIRV][DX] Share one test between backends (PR #65975)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

> Would it make the most sense to have a place like `test/Frontend/HLSL/` (to match `lib/Frontend/HLSL` - though there isn't such a directory yet)? This would provide a common place for HLSL specific constructs like numthreads and resources at least.

Yes, this makes sense. I don't know what the broader community will think of having frontend-specific tests here, but I do agree it would make sense.

https://github.com/llvm/llvm-project/pull/65975