Fix bug in map_fusion transformation

[edopao]

Four-lines bugfix and associated test case for map_fusion transformation.

Without this change, the test would fail in SDFG validation with error:
dace.sdfg.validation.InvalidSDFGEdgeError: Memlet data does not match source or destination data nodes) (at state state, edge __s0_n1None_n3IN_T[0] (V:None -> numeric:_inp))

[edopao]

@alexnick83 I can give you some context here. The problem addressed in this PR is different from the problem addressed in #1535. The problem addressed here blocks a delivery on gt4py main branch (for reference GridTools/gt4py#1502), which simplifies the icon4py SDFGs.

[alexnick83]

The fix looks correct for your use case, but I am unsure if it will always lead to correct results due to how it is applied. I explain in more detail in my comment. The fact that this bug exists in the first place shows that we just don't have enough tests for a lot of different cases regarding MapFusion. Therefore, I suggest that you limit the applicability of your fix until we can study related issues in more detail. Please find an example in my comment and let me know if it is something you can work with. If it is urgent, I can check again on short notice.

[edopao]

Thank you @alexnick83 for reviewing this PR. I have limited the application of this fix to the known valid case, as you suggested.
However, I skipped this line:
if current_data is not None and current_data not in test_data and list(edge.data.subset) == [0]:
because it is always true: the current_data is set at line 463-464.