borrowed from https://github.com/christianalfoni/react-bootstrap/blob/a7526e26a10359e802f5b1f6393131c687c3ea0e/test/DropdownButtonSpec.js

React_test_focused_state.js

it('should focus dropdown items when pressing cursor up and down keys', function () {

    // Have to run this in actual DOM to verify focus
    let testContainer = document.createElement('div');
    document.body.appendChild(testContainer);

    // Create instance as normal, only in DOM
    instance = React.render(
        <DropdownButton title="Title">
          <MenuItem eventKey="1">MenuItem 1 content</MenuItem>
          <MenuItem eventKey="2">MenuItem 2 content</MenuItem>
        </DropdownButton>
    , testContainer);

    let items = ReactTestUtils.scryRenderedDOMComponentsWithTag(instance, 'a');
    ReactTestUtils.Simulate.click(instance.refs.dropdownButton);

    let menu = React.findDOMNode(instance.refs.menu);

    ReactTestUtils.Simulate.keyDown(menu, {keyCode: 40});
    assert.equal(document.activeElement, items[0].getDOMNode());
    ReactTestUtils.Simulate.keyDown(menu, {keyCode: 40});
    assert.equal(document.activeElement, items[1].getDOMNode());
    ReactTestUtils.Simulate.keyDown(menu, {keyCode: 38});
    assert.equal(document.activeElement, items[0].getDOMNode());

    // Clean up
    React.unmountComponentAtNode(testContainer);
    document.body.removeChild(testContainer);

  });