There are lots of app features that can’t be tested quickly without changing the source code. Think of this typical task every developer has had to perform: three days after a user has registered, you want to offer them the option of purchasing premium access to the product at a discounted price. To test whether this promo is going to work, you need to register a test user and then wait three days until the promised dialog window appears. But clearly, a developer isn’t going to want to wait that long. …