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

[Nathan Gauër via llvm-commits]

Keenuts wrote:

One failure on presubmit: python formatting. From the logs it's a bad argument, infra related (https://github.com/llvm/llvm-project/actions/runs/6259826929/job/16996615363?pr=65975).
Unrelated.

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