Tests: extract TestHelpers.forceScrollableWindow for tests that need the window to scroll
Draggable Tests: make sure offset tests are actually testing offsets
Draggable Tests: clarify scroll helpers.
Draggable Tests: consolidate drag tests and provide error messages for all assertions.
Draggable Tests: Add position asserts to drag tests

They're currently disabled however due to bugs both
in the tests and in the actual widget.
Draggable Tests: Fix lint error.