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>
diff --git a/tools/dtoc/test/dtoc_test_simple.dts b/tools/dtoc/test/dtoc_test_simple.dts
index d8ab861..b5c1274 100644
--- a/tools/dtoc/test/dtoc_test_simple.dts
+++ b/tools/dtoc/test/dtoc_test_simple.dts
@@ -56,4 +56,8 @@
 			low-power;
 		};
 	};
+
+	orig-node {
+		orig = <1 23 4>;
+	};
 };