Add regression test coverage for timetz_izone().

Extend the test added by commit 97957fd so that it also covers
timetz_izone(), that is the "AT TIME ZONE interval" case.
This is mostly to see if xlc's apparent bug occurs there too,
but more code coverage is always welcome.

Discussion: https://postgr.es/m/2287835.1697464481@sss.pgh.pa.us

Author: tglsfdc

src/test/regress/expected/timetz.out

@@ -263,44 +263,45 @@ SELECT date_part('epoch',       TIME WITH TIME ZONE '2020-05-26 13:30:25.575401-
 (1 row)
 
 --
--- AT LOCAL with timetz
+-- Test timetz_zone, timetz_izone, AT LOCAL
 --
 BEGIN;
 SET LOCAL TimeZone TO 'UTC';
 CREATE VIEW timetz_local_view AS
   SELECT f1 AS dat,
        timezone(f1) AS dat_func,
        f1 AT LOCAL AS dat_at_local,
-       f1 AT TIME ZONE current_setting('TimeZone') AS dat_at_time
+       f1 AT TIME ZONE current_setting('TimeZone') AS dat_at_tz,
+       f1 AT TIME ZONE INTERVAL '00:00' AS dat_at_int
   FROM TIMETZ_TBL
   ORDER BY f1;
 SELECT pg_get_viewdef('timetz_local_view', true);
-                             pg_get_viewdef                             
-------------------------------------------------------------------------
-  SELECT f1 AS dat,                                                    +
-     timezone(f1) AS dat_func,                                         +
-     (f1 AT LOCAL) AS dat_at_local,                                    +
-     (f1 AT TIME ZONE current_setting('TimeZone'::text)) AS dat_at_time+
-    FROM timetz_tbl                                                    +
+                            pg_get_viewdef                             
+-----------------------------------------------------------------------
+  SELECT f1 AS dat,                                                   +
+     timezone(f1) AS dat_func,                                        +
+     (f1 AT LOCAL) AS dat_at_local,                                   +
+     (f1 AT TIME ZONE current_setting('TimeZone'::text)) AS dat_at_tz,+
+     (f1 AT TIME ZONE '@ 0'::interval) AS dat_at_int                  +
+    FROM timetz_tbl                                                   +
    ORDER BY f1;
 (1 row)
 
 TABLE timetz_local_view;
-      dat       |    dat_func    |  dat_at_local  |  dat_at_time   
-----------------+----------------+----------------+----------------
- 00:01:00-07    | 07:01:00+00    | 07:01:00+00    | 07:01:00+00
- 01:00:00-07    | 08:00:00+00    | 08:00:00+00    | 08:00:00+00
- 02:03:00-07    | 09:03:00+00    | 09:03:00+00    | 09:03:00+00
- 08:08:00-04    | 12:08:00+00    | 12:08:00+00    | 12:08:00+00
- 07:07:00-08    | 15:07:00+00    | 15:07:00+00    | 15:07:00+00
- 11:59:00-07    | 18:59:00+00    | 18:59:00+00    | 18:59:00+00
- 12:00:00-07    | 19:00:00+00    | 19:00:00+00    | 19:00:00+00
- 12:01:00-07    | 19:01:00+00    | 19:01:00+00    | 19:01:00+00
- 15:36:39-04    | 19:36:39+00    | 19:36:39+00    | 19:36:39+00
- 15:36:39-05    | 20:36:39+00    | 20:36:39+00    | 20:36:39+00
- 23:59:00-07    | 06:59:00+00    | 06:59:00+00    | 06:59:00+00
- 23:59:59.99-07 | 06:59:59.99+00 | 06:59:59.99+00 | 06:59:59.99+00
+      dat       |    dat_func    |  dat_at_local  |   dat_at_tz    |   dat_at_int   
+----------------+----------------+----------------+----------------+----------------
+ 00:01:00-07    | 07:01:00+00    | 07:01:00+00    | 07:01:00+00    | 07:01:00+00
+ 01:00:00-07    | 08:00:00+00    | 08:00:00+00    | 08:00:00+00    | 08:00:00+00
+ 02:03:00-07    | 09:03:00+00    | 09:03:00+00    | 09:03:00+00    | 09:03:00+00
+ 08:08:00-04    | 12:08:00+00    | 12:08:00+00    | 12:08:00+00    | 12:08:00+00
+ 07:07:00-08    | 15:07:00+00    | 15:07:00+00    | 15:07:00+00    | 15:07:00+00
+ 11:59:00-07    | 18:59:00+00    | 18:59:00+00    | 18:59:00+00    | 18:59:00+00
+ 12:00:00-07    | 19:00:00+00    | 19:00:00+00    | 19:00:00+00    | 19:00:00+00
+ 12:01:00-07    | 19:01:00+00    | 19:01:00+00    | 19:01:00+00    | 19:01:00+00
+ 15:36:39-04    | 19:36:39+00    | 19:36:39+00    | 19:36:39+00    | 19:36:39+00
+ 15:36:39-05    | 20:36:39+00    | 20:36:39+00    | 20:36:39+00    | 20:36:39+00
+ 23:59:00-07    | 06:59:00+00    | 06:59:00+00    | 06:59:00+00    | 06:59:00+00
+ 23:59:59.99-07 | 06:59:59.99+00 | 06:59:59.99+00 | 06:59:59.99+00 | 06:59:59.99+00
 (12 rows)
 
-DROP VIEW timetz_local_view;
-COMMIT;
+ROLLBACK;

src/test/regress/sql/timetz.sql

@@ -86,18 +86,18 @@ SELECT date_part('second',      TIME WITH TIME ZONE '2020-05-26 13:30:25.575401-
 SELECT date_part('epoch',       TIME WITH TIME ZONE '2020-05-26 13:30:25.575401-04');
 
 --
--- AT LOCAL with timetz
+-- Test timetz_zone, timetz_izone, AT LOCAL
 --
 BEGIN;
 SET LOCAL TimeZone TO 'UTC';
 CREATE VIEW timetz_local_view AS
   SELECT f1 AS dat,
        timezone(f1) AS dat_func,
        f1 AT LOCAL AS dat_at_local,
-       f1 AT TIME ZONE current_setting('TimeZone') AS dat_at_time
+       f1 AT TIME ZONE current_setting('TimeZone') AS dat_at_tz,
+       f1 AT TIME ZONE INTERVAL '00:00' AS dat_at_int
   FROM TIMETZ_TBL
   ORDER BY f1;
 SELECT pg_get_viewdef('timetz_local_view', true);
 TABLE timetz_local_view;
-DROP VIEW timetz_local_view;
-COMMIT;
+ROLLBACK;