Ensure plan stability in contrib/btree_gist regression test.

Buildfarm member skink failed with symptoms suggesting that an
auto-analyze had happened and changed the plan displayed for a
test query.  Although this is evidently of low probability,
regression tests that sometimes fail are no fun, so add commands
to force a bitmap scan to be chosen.

contrib/btree_gist/expected/not_equal.out

@@ -9,6 +9,7 @@ INSERT INTO test_ne SELECT '2009-01-01', 10.7 FROM generate_series(1,1000);
 INSERT INTO test_ne VALUES('2007-02-03', -91.3);
 INSERT INTO test_ne VALUES('2011-09-01', 43.7);
 INSERT INTO test_ne SELECT '2009-01-01', 10.7 FROM generate_series(1,1000);
+SET enable_indexscan to false;
 EXPLAIN (COSTS OFF) SELECT * FROM test_ne WHERE a <> '2009-01-01' AND b <> 10.7;
                                               QUERY PLAN                                              
 ------------------------------------------------------------------------------------------------------
@@ -25,6 +26,7 @@ SELECT * FROM test_ne WHERE a <> '2009-01-01' AND b <> 10.7;
  Thu Sep 01 00:00:00 2011 |  43.7
 (2 rows)
 
+RESET enable_indexscan;
 -- test search for "not equals" using an exclusion constraint
 CREATE TABLE zoo (
    cage   INTEGER,

contrib/btree_gist/sql/not_equal.sql

@@ -14,10 +14,14 @@ INSERT INTO test_ne VALUES('2007-02-03', -91.3);
 INSERT INTO test_ne VALUES('2011-09-01', 43.7);
 INSERT INTO test_ne SELECT '2009-01-01', 10.7 FROM generate_series(1,1000);
 
+SET enable_indexscan to false;
+
 EXPLAIN (COSTS OFF) SELECT * FROM test_ne WHERE a <> '2009-01-01' AND b <> 10.7;
 
 SELECT * FROM test_ne WHERE a <> '2009-01-01' AND b <> 10.7;
 
+RESET enable_indexscan;
+
 -- test search for "not equals" using an exclusion constraint
 
 CREATE TABLE zoo (