Further stabilize a postgres_fdw test case.

The buildfarm occasionally shows a variant row order in the output
of this UPDATE ... RETURNING, implying that the preceding INSERT
dropped one of the rows into some free space within the table rather
than appending them all at the end.  It's not entirely clear why that
happens some times and not other times, but we have established that
it's affected by concurrent activity in other databases of the
cluster.  In any case, the behavior is not wrong; the test is at fault
for presuming that a seqscan will give deterministic row ordering.
Add an ORDER BY atop the update to stop the buildfarm noise.

The buildfarm seems to have shown this only in v18 and master
branches, but just in case the cause is older, back-patch to
all supported branches.

Discussion: https://postgr.es/m/3866274.1770743162@sss.pgh.pa.us
Backpatch-through: 14

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -6292,20 +6292,31 @@ UPDATE ft2 d SET c2 = CASE WHEN random() >= 0 THEN d.c2 ELSE 0 END
 ALTER SERVER loopback OPTIONS (DROP extensions);
 INSERT INTO ft2 (c1,c2,c3)
   SELECT id, id % 10, to_char(id, 'FM00000') FROM generate_series(2001, 2010) id;
+-- this will do a remote seqscan, causing unstable result order, so sort
 EXPLAIN (verbose, costs off)
-UPDATE ft2 SET c3 = 'bar' WHERE postgres_fdw_abs(c1) > 2000 RETURNING *;            -- can't be pushed down
-                                                QUERY PLAN                                                
-----------------------------------------------------------------------------------------------------------
- Update on public.ft2
-   Output: c1, c2, c3, c4, c5, c6, c7, c8
-   Remote SQL: UPDATE "S 1"."T 1" SET c3 = $2 WHERE ctid = $1 RETURNING "C 1", c2, c3, c4, c5, c6, c7, c8
-   ->  Foreign Scan on public.ft2
-         Output: 'bar'::text, ctid, ft2.*
-         Filter: (postgres_fdw_abs(ft2.c1) > 2000)
-         Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8, ctid FROM "S 1"."T 1" FOR UPDATE
-(7 rows)
+WITH cte AS (
+  UPDATE ft2 SET c3 = 'bar' WHERE postgres_fdw_abs(c1) > 2000 RETURNING *
+) SELECT * FROM cte ORDER BY c1;          -- can't be pushed down
+                                                    QUERY PLAN                                                    
+------------------------------------------------------------------------------------------------------------------
+ Sort
+   Output: cte.c1, cte.c2, cte.c3, cte.c4, cte.c5, cte.c6, cte.c7, cte.c8
+   Sort Key: cte.c1
+   CTE cte
+     ->  Update on public.ft2
+           Output: ft2.c1, ft2.c2, ft2.c3, ft2.c4, ft2.c5, ft2.c6, ft2.c7, ft2.c8
+           Remote SQL: UPDATE "S 1"."T 1" SET c3 = $2 WHERE ctid = $1 RETURNING "C 1", c2, c3, c4, c5, c6, c7, c8
+           ->  Foreign Scan on public.ft2
+                 Output: 'bar'::text, ft2.ctid, ft2.*
+                 Filter: (postgres_fdw_abs(ft2.c1) > 2000)
+                 Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8, ctid FROM "S 1"."T 1" FOR UPDATE
+   ->  CTE Scan on cte
+         Output: cte.c1, cte.c2, cte.c3, cte.c4, cte.c5, cte.c6, cte.c7, cte.c8
+(13 rows)
 
-UPDATE ft2 SET c3 = 'bar' WHERE postgres_fdw_abs(c1) > 2000 RETURNING *;
+WITH cte AS (
+  UPDATE ft2 SET c3 = 'bar' WHERE postgres_fdw_abs(c1) > 2000 RETURNING *
+) SELECT * FROM cte ORDER BY c1;
   c1  | c2 | c3  | c4 | c5 | c6 |     c7     | c8 
 ------+----+-----+----+----+----+------------+----
  2001 |  1 | bar |    |    |    | ft2        | 

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1572,9 +1572,16 @@ UPDATE ft2 d SET c2 = CASE WHEN random() >= 0 THEN d.c2 ELSE 0 END
 ALTER SERVER loopback OPTIONS (DROP extensions);
 INSERT INTO ft2 (c1,c2,c3)
   SELECT id, id % 10, to_char(id, 'FM00000') FROM generate_series(2001, 2010) id;
+
+-- this will do a remote seqscan, causing unstable result order, so sort
 EXPLAIN (verbose, costs off)
-UPDATE ft2 SET c3 = 'bar' WHERE postgres_fdw_abs(c1) > 2000 RETURNING *;            -- can't be pushed down
-UPDATE ft2 SET c3 = 'bar' WHERE postgres_fdw_abs(c1) > 2000 RETURNING *;
+WITH cte AS (
+  UPDATE ft2 SET c3 = 'bar' WHERE postgres_fdw_abs(c1) > 2000 RETURNING *
+) SELECT * FROM cte ORDER BY c1;          -- can't be pushed down
+WITH cte AS (
+  UPDATE ft2 SET c3 = 'bar' WHERE postgres_fdw_abs(c1) > 2000 RETURNING *
+) SELECT * FROM cte ORDER BY c1;
+
 EXPLAIN (verbose, costs off)
 UPDATE ft2 SET c3 = 'baz'
   FROM ft4 INNER JOIN ft5 ON (ft4.c1 = ft5.c1)