[mlir][sparse][nfc] Use tensor.generate in sparse integration tests

Currently, dense tensors are initialized in Sparse Integration tests using
"buffer.tensor_alloc and scf.for" . This makes code harder to read and maintain.
This diff uses tensor.generate instead to initialize dense tensors.

Testing: Ran integration tests after building with -DLLVM_USE_SANITIZER=Address flag.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D131404