Update: flatten-array add additional test cases

[kwchang0831]

problem-specifications #1953

Implements the additional test cases.

[github-actions]

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• Any exercise changed

  • 👤 Does the author of the PR need to be added as an author or contributor in <exercise>/.meta/config.json (see docs)?
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• Practice exercise changed

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[angelikatyborska]

Looks perfect, thank you!