Add an integration test for PhoneBillDAO to improve code coverage

[DavidWhitlock]

A student pointed out that the main method of the PhoneBillDAO class is not currently covered by the out-of-the-box automated tests. As a result, it's more difficult to achieve the 75% overall project code coverage.

So, I should add an integration test for PhoneBillDAO that invokes the main method after creating a database file on disk.