Add tests to data_structures/linked_list/swap_nodes.py (#10751)

* Added doctests to the swap_nodes file under linkedlist data structure

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* Added doctests to the swap_nodes file under linkedlist data structure

* Added doctests to the swap_nodes file under linkedlist data structure

* Added doctests to the swap_nodes file under linkedlist data structure

* Update swap_nodes.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>