Add tests for WithDragAndDrop
Add tests for WithDragAndDrop

Create an `identity` function, then use this to mock `connectDragSource`
and `connectDropTarget` and test it has been called.

The name `identity` seems to be significant too. Calling the mock
function something other than `identity` led to console errors when
running the tests.

Sage/carbon,Sage/carbon,Sage/carbon