of: unittest: add caution to function header comment

Name of function attach_node_and_children() is misleading because
if the node already exists in the livetree then only the node's
properties are attached.  This works for the existing test data,
but add comment warning of this misleading name.

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