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

[Justin Bogner via llvm-commits]

Nathan =?utf-8?q?Gau=C3=ABr?= <brioche at google.com>,
Nathan =?utf-8?q?Gau=C3=ABr?= <brioche at google.com>


https://github.com/bogner approved this pull request.

This looks good to me. @llvm-beanz are you happy with this approach?

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