Fix intermittent failure on add_parent test

Closes #1511 (closed)

I cannot reproduce this at all locally, so it took me an hour to reproduce it in CI with some extra print(). I tested various methods of ordering the parent paths or making the query more deterministic, and with a bunch of EXPLAIN I found that adding ORDER BY path (not id or element_id or ordering or something else) caused the query plan to not change one bit, likely because the index is already internally ordered by path, so this should not waste the performance boosts that we got with the rewrite of add_parent.

@vrigal Since you seem to be able to reproduce more easily, can you check whether or not the failure seems to have been fixed on this branch?