matrix: add tests for zero indexes

[soxfox42]

I noticed that it's very easy to accidentally write a solution to "Matrix" that panics when given an index of 0, since unchecked subtraction can panic in debug builds, and posted about it on the forum.

I realise that this change may will affect old solutions, and I'm happy to discuss further on the forum if needed, but I figured I'd prepare this simple PR anyway.

[github-actions]

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[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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[no important files changed]

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