Attempt to stabilize postgres_fdw tests

cac169d68 adjusted DEFAULT_FDW_TUPLE_COST and that seems to have caused
a test to become unstable on 32-bit machines.  Try to make it stable
again.

Reported-by: Michael Paquier
Discussion: https://postgr.es/m/ZUM2IhA8X2lrG50K@paquier.xyz

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -4018,6 +4018,7 @@ EXECUTE st1(101, 101);
  00101 | 00101
 (1 row)
 
+SET enable_hashjoin TO off;
 -- subquery using stable function (can't be sent to remote)
 PREPARE st2(int) AS SELECT * FROM ft1 t1 WHERE t1.c1 < $2 AND t1.c3 IN (SELECT c3 FROM ft2 t2 WHERE c1 > $1 AND date(c4) = '1970-01-17'::date) ORDER BY c1;
 EXPLAIN (VERBOSE, COSTS OFF) EXECUTE st2(10, 20);
@@ -4052,6 +4053,7 @@ EXECUTE st2(101, 121);
  116 |  6 | 00116 | Sat Jan 17 00:00:00 1970 PST | Sat Jan 17 00:00:00 1970 | 6  | 6          | foo
 (1 row)
 
+RESET enable_hashjoin;
 -- subquery using immutable function (can be sent to remote)
 PREPARE st3(int) AS SELECT * FROM ft1 t1 WHERE t1.c1 < $2 AND t1.c3 IN (SELECT c3 FROM ft2 t2 WHERE c1 > $1 AND date(c5) = '1970-01-17'::date) ORDER BY c1;
 EXPLAIN (VERBOSE, COSTS OFF) EXECUTE st3(10, 20);

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1122,11 +1122,13 @@ PREPARE st1(int, int) AS SELECT t1.c3, t2.c3 FROM ft1 t1, ft2 t2 WHERE t1.c1 = $
 EXPLAIN (VERBOSE, COSTS OFF) EXECUTE st1(1, 2);
 EXECUTE st1(1, 1);
 EXECUTE st1(101, 101);
+SET enable_hashjoin TO off;
 -- subquery using stable function (can't be sent to remote)
 PREPARE st2(int) AS SELECT * FROM ft1 t1 WHERE t1.c1 < $2 AND t1.c3 IN (SELECT c3 FROM ft2 t2 WHERE c1 > $1 AND date(c4) = '1970-01-17'::date) ORDER BY c1;
 EXPLAIN (VERBOSE, COSTS OFF) EXECUTE st2(10, 20);
 EXECUTE st2(10, 20);
 EXECUTE st2(101, 121);
+RESET enable_hashjoin;
 -- subquery using immutable function (can be sent to remote)
 PREPARE st3(int) AS SELECT * FROM ft1 t1 WHERE t1.c1 < $2 AND t1.c3 IN (SELECT c3 FROM ft2 t2 WHERE c1 > $1 AND date(c5) = '1970-01-17'::date) ORDER BY c1;
 EXPLAIN (VERBOSE, COSTS OFF) EXECUTE st3(10, 20);