This commit fixes a bug (or feature? unclear at this stage) of the
`rootOf` function inside `searchTableNgrams`, which could, in some
occasion, return a root which has a different `ListType`, which
in turn throws off pagination.

This commit adds some basic tests for the Ngrams pagination,
capitalizing on the refactoring of the `searchTableNgrams` function,
which is now pure and can be tested fairly easily.

This commit refactors the Ngrams pagination code to use
the `NgramsSearchQuery`, and it introduces a new function called
`searchTableNgrams`, which is now completely pure.

This commit partially rewrites the `testPrios` queue test, in such a way
that it's now easier to inspect the final schedule and the occurrence of events.
Albeit not perfect, it should be less sensitive to random failures,
like the ones we have been observing on CI.