class LoginPage extends Component {
    constructor(props) {
        super(props);
        this.state = {
            username: '',
            password: ''
        };

        this.handleChange = this.handleChange.bind(this);

        ...
    }

    handleChange(event) {
        this.setState({[event.target.name]: event.target.value})
    }

    ...

    return(
        ...
        <TextField name='username' onChange={this.handleChange} />
    )

it('handles change', () => {
    const spy = sinon.spy(LoginPage.prototype, 'handleChange');
    const _wrapper = shallow(<LoginPage />);

    spy({
            event: {
                target: {
                    name: 'username',
                    value: 'password'
                }
            }
        }
    );

    expect(_wrapper.state().username).to.equal('username');
    expect(_wrapper.state().password).to.equal('password')
})