test: add missing tests for max-affected preference with rpc

test/spec/fixtures/schema.sql

@@ -3815,3 +3815,8 @@ set oid = 'test.collision_test_table'::regclass::oid
 where oid = 'test.collision_test_func'::regproc::oid;
 
 comment on function test.collision_test_func(id integer) is 'fizzbuzz';
+
+
+create or replace function test.delete_all_items() returns setof items as $$
+  delete from items where id <= 15 returning *; -- deletes 15 items, then return them
+$$ language sql;