difference-of-squares: Sync tests

[tasxatzial]

Also updated the difference_of_squares.clj starter file with a proper function template

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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[tasxatzial]

Thanks Isaac. Any idea why it still shows merging blocked?

[IsaacG]

@bobbicodes @michalporeba Could one of you, as track maintainers, approve?

[IsaacG]

I think it's blocked on a maintainer approval