of: unittest: remove report of expected error

update_node_properties() reports an error when the test data contains
a node (such as "/aliases") that already exists in the base devicetree.
The error is caused by of_fdt_unflatten_tree() autogenerating the
"name" property, thus both the existing node and the new node will
have a property with the same name.  Suppress reporting the known
error.

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