of: overlay: unittest: add tests for overlay notifiers

Add tests for overlay apply and remove notifiers.  Trigger errors
for each of the notifier actions.

These tests will reveal a memory leak problem when a notifier returns
an error for action OF_OVERLAY_POST_APPLY.  The pr_err() message is:

   OF: ERROR: memory leak, expected refcount 1 instead of 3,
   of_node_get()/of_node_put() unbalanced - destroy cset entry: attach
   overlay node /testcase-data/overlay-node/test-bus/test-unittest17

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220502181742.1402826-3-frowand.list@gmail.com