dillon1000/react

Let components specify tag for ReactTestUtils.mockComponent to use.

If you're writing a test that involves a mocked component that normally
returns a `<li>` tag from its `render` method as the root element, mocking
it with a dummy `<div>` probably won't work, so you'll want to set

  MyListItemComponent.mockTagName = 'li';

The change to the `.mockImplementation` line makes sense because
`ConvenienceConstructor` is more or less synonymous (for these purposes)
with the wrapper that was previously used:

  function() {
    // This `this` used to be `null`, technically, but
    // ConvenienceConstructor doesn't pay attention to `this` anyway.
    return ConvenienceConstructor.apply(this, arguments);
  }