The created entity node's reference count needs to be decremented to 0
before it is added as a child, because appendChild will increment the
reference count to correct value of 1. Re-enabled commented-out test data
tst_QDom::setContent to exercise the code path with the leak.
Change-Id: Ieb015d68ba9bbb3f20dd47e76835ad15abb1738e
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>