DROP OWNED BY has a specific code path to remove ACLs stored in
pg_default_acl when cleaning up shared dependencies that had no
coverage with the existing tests.  This issue has been found while
digging into the bug fixed by 21378e1f.

As ALTER DEFAULT PRIVILEGES impacts the ACLs of all objects created
while the default permissions are visible, the test uses a transaction
rollback to isolate the test and avoid any impact with other sessions
running in parallel.

Reviewed-by: Álvaro Herrera
Discussion: https://postgr.es/m/YAbQ1OD+3ip4lRv8@paquier.xyz