To integrate with an external web service using HTTP requests and set up a testing mechanism using the Service Framework in Salesforce B2C Commerce, the best approach is to implement the mockFill callback and set the service mode to "mock". This configuration allows the developer to test the integration with a simulated response from the service. By using the mockFill method, developers can specify the expected response from the external service when it is not yet available. This approach is particularly useful for testing and debugging the integration in a controlled environment before the actual web service goes live. Setting the service mode to "mock" enables this simulated environment, ensuring that the service behaves as if it is interacting with the actual external service.