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

One big issue with DirectXShaderCompiler was test coverage: DXIL and
SPIR-V backends had their own tests. When a bug was found in one, the
other wasn't always checked. This lead to unequal support of HLSL for
both backends. We'd like to avoid those issues here, hence the
test-sharing.

By default, all the tests in this folder are marked as requiring
DirectX. But as SPIR-V support grows, each test drop this requirement,
and check the SPIR-V behavior.

I would have preferred to mark new tests as XFAIL for SPIR-V by default,
so we could differentiate real unsupported tests (as SPIR-V has no
equivalent), from newly added tests. But the way LIT is built, I don't
think this is possible.

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Signed-off-by: Nathan Gauër <brioche@google.com>