[feature] Add test codes

## Issue Type
- [ ] Bug report
- [ V ] Feature request
- [ ] Documentation update
- [ ] Other (please specify)

## Description
Currently, the codes of the game do not have any test codes. This issue proposes to add structured test cases for each code to improve reliability, support future refactoring, and ensure feature integrity.

## Expected behaviour
Each major module should have its own unit test file that:
- Verifies class initialization (`Game`)
- Validates asset loading behavior
- Confirms visual rendering logic does not raise errors during execution

## Actual behaviour
No test files exist for the codes. All logic currently runs without automated validation.

## Your Environment (for bugs):
- Operating System: Window 11