Fixed DragAndDropWrapper using wrong drop target in IE8 #12406

VDragAndDropManager was assuming that the target element will always be
inside the cloned "drag image" element while dragging. This assumption
is false since the "drag image" can be 0x0px or transparent effectivly
disabling dragging.

Since Testbench 2 is also very flaky in using the Vaadin locators with
the drag/drop commands I replaced the locators with shorter locators
using a debug id to make the test more readable and stable.

Change-Id: I2cc9683d11e982521e74418c74dd3e81ee617ac5