The recovery conflict tests added in 9f8a050f68 surfaced a bug in the
interaction between buffer pin and deadlock recovery conflicts. To make sure
that the bugfix won't break deadlock conflict detection, add a test for that
scenario.
031_recovery_conflict.pl will later be backpatched, with this included.
Discussion: https://postgr.es/m/20220413002626.udl7lll7f3o7nre7@alap3.anarazel.de