Update difference-of-squares tests

[BNAndras]

Adds two missing tests as well as a stub for the student.

[github-actions[bot]]

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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[BNAndras]

I think I ran afoul of how the test runner pastes the example into the stub where the tests are defined. I'll troubleshoot later this week.