dm: core: Expand ofnode tests

The current tests do not cover all functions, nor do they cover the new
multi-tree functionality. Add and update the tests accordingly and update
the 'future work' notes in the documentation.

There is a still more testing needed for the failure cases, since at
present some ofnode functions return a libfdt error code instead of
converting it to an errno.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/driver-model/livetree.rst b/doc/develop/driver-model/livetree.rst
index 76be89b..65b88f8 100644
--- a/doc/develop/driver-model/livetree.rst
+++ b/doc/develop/driver-model/livetree.rst
@@ -321,10 +321,7 @@
 Future work
 -----------
 
-Live tree support was introduced in U-Boot 2017.07. There is still quite a bit
-of work to do to flesh this out:
+Live tree support was introduced in U-Boot 2017.07. Some possible enhancements
+are:
 
-- tests for all access functions
-- more support for livetree modification
-- addition of more access functions as needed
 - support for livetree in SPL and before relocation (if desired)