Add two more unittests for Node add/remove

These just test that add/remove are idempotent. This is not perfect,
as we use unsorted lists for some values (instead of sets), so when
using non-empty nodes this would break (but for empty nodes, a
1-element list is sorted, so it's fine).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>