fix: add doctest.testmod() call to gnome_sort main block

[mithunbabbira]

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Fixes #14881

The __main__ block of sorts/gnome_sort.py was missing the doctest.testmod() call, unlike the other sorting files in the same directory (e.g. insertion_sort.py). Added it following the existing convention so doctests run when the file is executed directly.

Verified: python3 -m doctest sorts/gnome_sort.py -v — all 4 doctests pass.