Add INSERT ON CONFLICT test on partitioned tables with transition table

This case was uncovered by existing tests, so breakage went undetected.
Make sure it remains stable.

Extracted from a larger patch by
Author: David Rowley
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/CAKJS1f-aGCJ5H7_hiSs5PhWs6Obmj+vGARjGymqH1=o5PcrNnQ@mail.gmail.com

src/test/regress/expected/insert_conflict.out

@@ -904,4 +904,26 @@ select * from parted_conflict order by a;
  50 | cincuenta | 2
 (1 row)
 
+-- test with statement level triggers
+create or replace function parted_conflict_update_func() returns trigger as $$
+declare
+    r record;
+begin
+ for r in select * from inserted loop
+	raise notice 'a = %, b = %, c = %', r.a, r.b, r.c;
+ end loop;
+ return new;
+end;
+$$ language plpgsql;
+create trigger parted_conflict_update
+    after update on parted_conflict
+    referencing new table as inserted
+    for each statement
+    execute procedure parted_conflict_update_func();
+truncate parted_conflict;
+insert into parted_conflict values (0, 'cero', 1);
+insert into parted_conflict values(0, 'cero', 1)
+  on conflict (a,b) do update set c = parted_conflict.c + 1;
+NOTICE:  a = 0, b = cero, c = 2
 drop table parted_conflict;
+drop function parted_conflict_update_func();

src/test/regress/sql/insert_conflict.sql

@@ -576,4 +576,30 @@ insert into parted_conflict values (50, 'cincuenta', 2)
 -- should see (50, 'cincuenta', 2)
 select * from parted_conflict order by a;
 
+-- test with statement level triggers
+create or replace function parted_conflict_update_func() returns trigger as $$
+declare
+    r record;
+begin
+ for r in select * from inserted loop
+	raise notice 'a = %, b = %, c = %', r.a, r.b, r.c;
+ end loop;
+ return new;
+end;
+$$ language plpgsql;
+
+create trigger parted_conflict_update
+    after update on parted_conflict
+    referencing new table as inserted
+    for each statement
+    execute procedure parted_conflict_update_func();
+
+truncate parted_conflict;
+
+insert into parted_conflict values (0, 'cero', 1);
+
+insert into parted_conflict values(0, 'cero', 1)
+  on conflict (a,b) do update set c = parted_conflict.c + 1;
+
 drop table parted_conflict;
+drop function parted_conflict_update_func();