Avoid row-processing-order dependency in postgres_fdw regression test.

A test intended to provoke an error on the remote side was coded in such
a way that multiple rows should be updated, so the output would vary
depending on which one was processed first.  Per buildfarm.

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -2142,7 +2142,7 @@ select c2, count(*) from ft2 where c2 < 500 group by 1 order by 1;
 (13 rows)
 
 savepoint s3;
-update ft2 set c2 = -2 where c2 = 42; -- fail on remote side
+update ft2 set c2 = -2 where c2 = 42 and c1 = 10; -- fail on remote side
 ERROR:  new row for relation "T 1" violates check constraint "c2positive"
 DETAIL:  Failing row contains (10, -2, 00010_trig_update_trig_update, 1970-01-11 08:00:00+00, 1970-01-11 00:00:00, 0, 0         , foo).
 CONTEXT:  Remote SQL command: UPDATE "S 1"."T 1" SET c2 = $2 WHERE ctid = $1

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -337,7 +337,7 @@ select c2, count(*) from ft2 where c2 < 500 group by 1 order by 1;
 release savepoint s2;
 select c2, count(*) from ft2 where c2 < 500 group by 1 order by 1;
 savepoint s3;
-update ft2 set c2 = -2 where c2 = 42; -- fail on remote side
+update ft2 set c2 = -2 where c2 = 42 and c1 = 10; -- fail on remote side
 rollback to savepoint s3;
 select c2, count(*) from ft2 where c2 < 500 group by 1 order by 1;
 release savepoint s3;