Write a program that simulates a basic e-commerce shopping cart. The program should include the following functionalities:

1. Add Item: Allow the user to add an item to the cart by specifying the product name and quantity.
2. Remove Item: Enable the user to remove an item from the cart by specifying the product name.
3. Update Quantity: Permit the user to update the quantity of a specific item in the cart.
4. Calculate Total: Compute and display the total cost of the items in the cart, assuming each item has a predefined price.
5. Checkout: Simulate the checkout process where the user can finalize their purchase and clear the cart.

Requirements:

• Use object-oriented programming principles to design the shopping cart.
• Implement error handling for cases such as attempting to remove an item that is not in the cart or updating the quantity to a negative number.
• Write unit tests to verify the functionality of the cart operations.

Sample Code Structure:

class ShoppingCart:
    def __init__(self):
        # Your code here

    def add_item(self, product_name, quantity, price):
        # Your code here

    def remove_item(self, product_name):
        # Your code here

    def update_quantity(self, product_name, quantity):
        # Your code here

    def calculate_total(self):
        # Your code here

    def checkout(self):
        # Your code here

# Example usage:
cart = ShoppingCart()
cart.add_item("Laptop", 1, 1000)
cart.add_item("Headphones", 2, 150)
print(cart.calculate_total())
cart.checkout()