Fix memory leak in QDomDocument DTD notation declaration handler

Fix memory leak in QDomDocument DTD notation declaration handler

The created notation 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. Also added autotest DTDNotationDecl
to tst_qdom to expose the leak when executed under valgrind memcheck.
There was no previous test coverage for the notation declarations in DTD.

Task-number: QTBUG-22588
Change-Id: I876186d1277ceb4414f803b58b62f51cc1474367
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>