dtoc: Add a subnode test for multiple nodes

Add a new test that adds a subnode alongside an existing one, as well as
adding properties to a subnode. This will expand to adding multiple
subnodes in future patches. Put a node after the one we are adding to so
we can check that things sync correctly.

The testAddNode() test should be in the TestNode class since it is a node
test, so move it.

Signed-off-by: Simon Glass <sjg@chromium.org>