Add tests and cleanup sum_of_subsets algorithm (#12746)

* Add tests and cleanup sum_of_subsets algorithm.

* Update sum_of_subsets.py

* Update sum_of_subsets.py

* Update sum_of_subsets.py

* Update sum_of_subsets.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>