Fix remaining tests

In particular:

* In ngrams search tests where we were comparing the total number of
  results returned, we need to account only for the roots, because the
  total number we would be returning from the pagination would be
  higher, as it would include children, in some cases;
* In the `AcyclipTableMap` generator, the roots we generate need to be
  so (i.e. `_ne_root` and `_ne_parent` must be `Nothing`).