gistfile1.js

// given this class
class LoginForm extends Component {
  static propTypes = {
    login: PropTypes.func,
  };

  render() {
    <Formik onSubmit={v => this.props.login(v.username, v.password)}>
      {({ handleSubmit }) => (
        <form onSubmit={handleSubmit}>
          {/* ... */}
          <button type="submit">submit</button>
        </form>
      )}
    </Formik>
  }
}

// in the test suite
import LoginForm from 'components/LoginForm';
import { spy } from 'sinon';
import { mount } from 'enzyme';

it('should call login when form is submitted', function() {
  // a spy is a function that tracks how many times its been called
  // see https://sinonjs.org/ for more info
  const loginSpy = spy();

  // instead of the real login function, pass in the spy
  const wrapper = mount(<LoginForm login={loginSpy} />);

  // simulate the form submission
  const submitBtn = wrapper.find('[type="submit"]');
  submitBtn.simulate('submit');

  // check to see that the login function was called on submit
  expect(loginSpy.calledOnce).toBeTruthy();
});