test: Add edge case tests for YearsBeforeDesiredBalance in Interest_Is_Interesting

[KaranChadha10]

Description

This PR contains the following points:

• Add test case for when target balance equals current balance
• Add test case for when target balance is already exceeded
• Both cases should return 0 years as target is already met
• Prevents incorrect do-while implementations from passing tests

Fixes #1976

[github-actions]

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[github-actions]

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[KaranChadha10]

@ErikSchierboom This PR was auto-closed by GitHub Actions, but I believe the issue has been properly resolved. Could you please review the changes when you have a chance?
The PR addresses the issue #1976.
Thank you!