test: add tests for NumberOfSubsetEqualToGivenSum

[vil02]

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Describe your change:

I added tests for NumberOfSubsetEqualToGivenSum. The current implementation does not handle properly inputs less or equal 0, so I decided to explicitly throw an error for such inputs.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.