feat: add ExpenseListController validation and authorization tests

src/main/java/de/zendric/app/xpensely_server/controller/ExpenseListController.java

@@ -19,7 +19,7 @@ import de.zendric.app.xpensely_server.services.UserService;
 
 @RestController
 @RequestMapping("/api/expenselist")
-class ExpenseListController {
+public class ExpenseListController {
 
     private final ExpenseListService expenseListService;
     private final UserService userService;