Back-patch test cases for timetz_zone/timetz_izone.

Per code coverage reports, we had zero regression test coverage
of these functions.  That came back to bite us, as apparently
that's allowed us to miss discovering misbehavior of this code
with AIX's xlc compiler.  Install relevant portions of the
test cases added in 97957fdba, 2f0472030, 19fa97731.

(Assuming the expected outcome that the xlc problem does appear
in back branches, a code fix will follow.)

Discussion: https://postgr.es/m/CA+hUKGK=DOC+hE-62FKfZy=Ybt5uLkrg3zCZD-jFykM-iPn8yw@mail.gmail.com

src/test/regress/expected/timetz.out

@@ -231,3 +231,63 @@ SELECT date_part('epoch',       TIME WITH TIME ZONE '2020-05-26 13:30:25.575401-
  63025.575401
 (1 row)
 
+--
+-- Test timetz_zone, timetz_izone
+--
+BEGIN;
+SET LOCAL TimeZone TO 'UTC';
+CREATE VIEW timetz_local_view AS
+  SELECT f1 AS dat,
+       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 timetz_tbl.f1 AS dat,                                                   +
+     (timetz_tbl.f1 AT TIME ZONE current_setting('TimeZone'::text)) AS dat_at_tz,+
+     (timetz_tbl.f1 AT TIME ZONE '@ 0'::interval) AS dat_at_int                  +
+    FROM timetz_tbl                                                              +
+   ORDER BY timetz_tbl.f1;
+(1 row)
+
+TABLE timetz_local_view;
+      dat       |   dat_at_tz    |   dat_at_int   
+----------------+----------------+----------------
+ 00:01:00-07    | 07:01:00+00    | 07:01:00+00
+ 01:00:00-07    | 08:00:00+00    | 08:00:00+00
+ 02:03:00-07    | 09:03:00+00    | 09:03:00+00
+ 08:08:00-04    | 12:08:00+00    | 12:08:00+00
+ 07:07:00-08    | 15:07:00+00    | 15:07:00+00
+ 11:59:00-07    | 18:59:00+00    | 18:59:00+00
+ 12:00:00-07    | 19:00:00+00    | 19:00:00+00
+ 12:01:00-07    | 19:01:00+00    | 19:01:00+00
+ 15:36:39-04    | 19:36:39+00    | 19:36:39+00
+ 15:36:39-05    | 20:36:39+00    | 20:36:39+00
+ 23:59:00-07    | 06:59:00+00    | 06:59:00+00
+ 23:59:59.99-07 | 06:59:59.99+00 | 06:59:59.99+00
+(12 rows)
+
+SELECT f1 AS dat,
+       f1 AT TIME ZONE 'UTC+10' AS dat_at_tz,
+       f1 AT TIME ZONE INTERVAL '-10:00' AS dat_at_int
+  FROM TIMETZ_TBL
+  ORDER BY f1;
+      dat       |   dat_at_tz    |   dat_at_int   
+----------------+----------------+----------------
+ 00:01:00-07    | 21:01:00-10    | 21:01:00-10
+ 01:00:00-07    | 22:00:00-10    | 22:00:00-10
+ 02:03:00-07    | 23:03:00-10    | 23:03:00-10
+ 08:08:00-04    | 02:08:00-10    | 02:08:00-10
+ 07:07:00-08    | 05:07:00-10    | 05:07:00-10
+ 11:59:00-07    | 08:59:00-10    | 08:59:00-10
+ 12:00:00-07    | 09:00:00-10    | 09:00:00-10
+ 12:01:00-07    | 09:01:00-10    | 09:01:00-10
+ 15:36:39-04    | 09:36:39-10    | 09:36:39-10
+ 15:36:39-05    | 10:36:39-10    | 10:36:39-10
+ 23:59:00-07    | 20:59:00-10    | 20:59:00-10
+ 23:59:59.99-07 | 20:59:59.99-10 | 20:59:59.99-10
+(12 rows)
+
+ROLLBACK;

src/test/regress/sql/timetz.sql

@@ -77,3 +77,23 @@ SELECT date_part('microsecond', TIME WITH TIME ZONE '2020-05-26 13:30:25.575401-
 SELECT date_part('millisecond', TIME WITH TIME ZONE '2020-05-26 13:30:25.575401-04');
 SELECT date_part('second',      TIME WITH TIME ZONE '2020-05-26 13:30:25.575401-04');
 SELECT date_part('epoch',       TIME WITH TIME ZONE '2020-05-26 13:30:25.575401-04');
+
+--
+-- Test timetz_zone, timetz_izone
+--
+BEGIN;
+SET LOCAL TimeZone TO 'UTC';
+CREATE VIEW timetz_local_view AS
+  SELECT f1 AS dat,
+       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;
+SELECT f1 AS dat,
+       f1 AT TIME ZONE 'UTC+10' AS dat_at_tz,
+       f1 AT TIME ZONE INTERVAL '-10:00' AS dat_at_int
+  FROM TIMETZ_TBL
+  ORDER BY f1;
+ROLLBACK;