Blacklist task256322_highlight on OS X

The test keeps failing. The approach of sending mouse move events is
inherently fragile due to the use of QCursor::setPos and the expectation
that that produces the correct sequence of mouse move events.

Change-Id: I07ec75460b70c27152e8775deffcb77fa9328d0c
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>