[FEATURE REQUEST] Add tests to the Flutter project.

[NeiHau]

Issue Reference: #26 - [FEATURE REQUEST] Add tests to the Flutter project

Summary: I have addressed the issue by creating unit tests to verify the functionality of the methods within the DatabaseService. These tests ensure that the critical components of the database interactions are functioning correctly, helping to prevent any potential issues before deployment.

Details of the Fix: add unit test for databaseService and add initialization method in main function.

Issue Number: #26 Changes Implemented: Added comprehensive unit tests covering various methods in the DatabaseService class. These tests check for correct data retrieval, insertion, updates, and deletion to ensure the integrity of the database operations.

Outcome: The newly added tests provide confidence that the database services are performing as expected, reducing the risk of bugs in the production environment. Please review the changes and let me know if any further adjustments are needed.