[mlir][sparse] Reorganize tests for the sparse_tensor.convert operator.

Rename conversion_sparse2dense.mlir and conversion_sparse2sparse.mlir to
convert_sparse2dense.mlir/sparse2sparse.mlir.

Add convert_dense2sparse.mlir. Move the sparse_tensor.convert operator tests
out of conversion.mlir.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D135922