[llvm] [mlir][sparse] Change tests to use new syntax for ELL and slice (PR #67569)

[Aart Bik via llvm-commits]

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@@ -240,8 +240,9 @@ def SparseTensorEncodingAttr : SparseTensor_Attr<"SparseTensorEncoding",
     // CSR slice (offset = 0, size = 4, stride = 1 on the first dimension;
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aartbik wrote:

Can we also add the #ELL here, so we can explain the "free" symbol (for now, since we may implement this very differently)

https://github.com/llvm/llvm-project/pull/67569