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>