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); }