of: unittest: clean up changeset test

In preparation for fixing __of_node_dup(), clean up the unittest
function that calls it.

Devicetree nodes created from a flattened device tree have a name
property.  Follow this convention for nodes added by a changeset.

For node added by changeset, remove incorrect initialization of
child node pointer.

Add an additional node pointer 'changeset' to more naturally reflect
where in the tree the changeset is added.

Make changeset add property error messages unique.

Add whitespace to break apart logic blocks.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>