Update tests flatten array

[jagdish-15]

Pull Request

This PR syncs tests.toml with the problem-specifications repository and updates the test file.

[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.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

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[SleeplessByte]

Great.

I feel that we don't need to re-run the tests on this one for the few people who passed the big null list and not the smaller ones, which would be a weird solution to begin with.