Update majority_element.cpp

[djeada]

No description provided.

[Copilot]

Pull Request Overview

Updates the majority element implementation by replacing the quickselect-based approach with a frequency counting method using hash maps, while also refactoring the testing infrastructure.

• Removes the quickselect partition-based majority element algorithm and replaces it with a hash map frequency counting approach
• Refactors testing infrastructure to use a generic template function for running tests
• Removes example comments and random number generation dependencies

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