Test a form with Jest and React JS TestUtils
The problem is that you are replacing the function after you have already given the original function to React. The expression onSubmit={this.done} is that function, and that is set as the event handler. After the render function finishes, you replace instance.done but React already got the old function. What you should do is instead:
<form className="myForm" onSubmit={() => this.done()}>
This makes sure that the event handler always invokes the method on the instance (the one you replaced). This has the nice side effect of being future compatible with React, since they will stop autobinding all methods to the instance.