RPL bugfix: don't re-add a bad parent

When RPL detects that a parent becomes really bad, as indicated by the ETX increasing to more than `dag->instance->max_rankinc`, the parent gets removed from being a parent. But then RPL will go through the list of parents and add the neighbor again because the `best_parent()` function doesn't check that the ETX is lower than `max_rankinc`. This patch adds this check to `best_parent()` and provides a regression test that triggers if a lost parent gets re-added.
Also noticing now that `acceptable_rank` doest not check that `dag->instance` is not null, as I think it should