Add tests without modifying code (#10740)

* Contributes to #9943
Added doctest to largest_of_very_large_numbers.py
Added doctest to word_patterns.py
Added doctest to onepad_cipher.py

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

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* Contributes to #9943
Added doctest to maths/largest_of_very_large_numbers.py
Added doctest to strings/word_patterns.py
Added doctest to ciphers/onepad_cipher.py

* Add tests without modifying code #10740
Added test to maths/largest_of_very_large_numbers
Added test to strings/word_patterns.py
Added test to ciphers/onepad_cipher.py

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