BUG: Fixed node reference counting tests

MRMLCreateNodeByClassWithSetReferenceCountToOne: removed. This test performs a double-delete and expects to get a runtime error. However, result of double-delete is "undefined behavior", i.e., it is not guaranteed to generate run-time error (and in some cases it does not).

MRMLCreateNodeByClassWithSetReferenceCountMinusOne: comments clarified.

1. Decreasing the reference count is needed because Python variable takes ownership of the created object, so the additional reference that Create...() adds is not needed anymore.
2. To decrement reference count, UnRegister() or Delete() method has to be called (SetReferenceCount should never be called).

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25286 3bd1e089-480b-0410-8dfb-8563597acbee