As bug #15733 has proved, we are lacking coverage for partition tuple
routing with dropped attributes when involving three levels of
partitioning or more.  There was only an active bug in this area for
v11, and HEAD is proving to handle those scenarios fine, still it lacked
some coverage for the previous problem.

Author: Amit Langote, Michael Paquier
Discussion: https://postgr.es/m/15733-7692379e310b80ec@postgresql.org