[Improvement] Creating test cases for everything

[Lei-Tin]

Abstractly

As stated in the Milestone 4 Rubric reviews, we are required to implement test cases for majority of our code (not all).

As of now, we only have test cases for MySQLDatabaseGateway.

We can try to include some for the test cases, some sample will be provided by @ScottCTD in a few days (?)

Actually

Remember that the test cases should not be too dependent on the other classes that also requires testing.

But reality is, most of our use cases depend on MySQLDatabaseGateway to make sure everything works, so I'll try my best to ensure that it works normally

TODOs

• [+] Added Unit tests for MySQLDatabaseGateway & Extra modifications within the class #31
• [+] Added tests for BillUpdateUseCase. #96