Improve test coverage

Added tests to some untested methods in FileHandlingBudget and GeneralBudget

src/main/java/no/ntnu/idatt1002/demo/controller/AddBudgetController.java

@@ -9,7 +9,6 @@ import javafx.scene.control.Button;
 import javafx.scene.control.ComboBox;
 import javafx.scene.control.Label;
 import javafx.scene.control.TextField;
-
 import javafx.scene.text.Text;
 import javafx.stage.Stage;
 import no.ntnu.idatt1002.demo.data.Budget.BudgetItem;