Support SNS MockNode creation in testutils

I'm trying to use MockNode of hippo-repository-testutils. It throws an
exception when adding SNS child. I see the following in MockNode.java:

public void addNode(MockNode child) {
child.setParent(this);
String childName = child.getName();
if (children.containsKey(childName))

{ throw new UnsupportedOperationException("Cannot add node '" + childName + "': MockNode does not support same-name siblings"); }

children.put(childName, child);
}

This doesn't look quite right to me. I think we should allow to add SNS
in MockNode by default and we'd better add methods like
MockNode#setSameNameSiblingSupported() and
MockNode#isSameNameSiblingSupported(). Also, static methods can be useful:
e.g, MockNode#setDefaultSameNameSiblingSupported() and
#isDefaultSameNameSiblingSupported().