如果您从不抛出异常，则您的测试将永远不会失败：）
这是我会重构你的代码：

class Shop {
 public void sell(User myUser, String item) {
    if(myUser.getRole() == Roles.CASHIER) {
        for(Product inventory : inventories) {
            if(inventory.getName().equals(item) && inventory.getPrice() <= myUser.getBalance() ) {
                System.out.println("sale successful");
                inventories.remove(inventory);
            }
            else {
                throw new OutOfStockException(item);
            }
        }
    } else {
        throw new CantSellException(myUser);
    }
  }
}

现在我们可以测试一个有效的销售它：

@Test
public void testValidSale() {
   User cashier = new User(Roles.CASHIER); // Create a valid User
   List<String> inventories = new ArrayList<>();
   inventories.add("Vodka");
   inventories.add("Beer");
   inventories.add("Rum");
   Shop shop = new Shop();
   shop.sell(cashier, "Vodka"); // An item that exists
   assertEquals(inventories.size(), 2);
   assertFalse(inventories.contains("Vodka"));
   // You should assert that inventories contains exactly "Beer" and "Rum"
}

@Test
public void testOutOfStockException() {
   User cashier = new User(Roles.CASHIER); // Create a valid User
   List<String> inventories = new ArrayList<>();
   inventories.add("Vodka");
   inventories.add("Beer");
   inventories.add("Rum");
   Shop shop = new Shop();
   try {
       shop.sell("whiskey"); // An item that's not in the inventory
       Assert.fail("We expect an exception when the item is out of stock");
   }
   catch(OutOfStockException e) {
       // Nothing to do: you can test that the error message is correct
       // if you want to
   }
}

public  void testUserCannotSell() {
  User customer = new User(Roles.CUSTOMER); // A user that cannot sell
   List<String> inventories = new ArrayList<>();
   inventories.add("Vodka");
   Shop shop = new Shop();
   try {
       // This item is not in the inventory
       // but the user cannot sell anything so the other exception should
       // happen first
        shop.sell("whiskey"); 
       Assert.fail("We expect an exception when the item is out of stock");
   }
   catch(CantSellException e) {
       // Nothing to do: you can test that the error message is correct
       // if you want to
   }

}

这段代码并不优雅，但它应该给予您了解测试方法的意义。