Fixed test case to be repeatable (after discussion with Igor)

mysql-test/r/index_merge_innodb.result

@@ -686,7 +686,7 @@ INSERT INTO t1 VALUES (1000000, 0, 0);
 SET SESSION sort_buffer_size = 1024*36;
 EXPLAIN
 SELECT COUNT(*) FROM 
-(SELECT * FROM t1 
+(SELECT * FROM t1 FORCE INDEX(primary,idx)
 WHERE a BETWEEN 2 AND 7 OR pk=1000000) AS t;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away

mysql-test/t/index_merge_innodb.test

@@ -64,9 +64,11 @@ INSERT INTO t1 VALUES (1000000, 0, 0);
 
 SET SESSION sort_buffer_size = 1024*36;
 
+# We have to use FORCE INDEX here as Innodb gives inconsistent estimates
+# which causes different query plans.
 EXPLAIN
 SELECT COUNT(*) FROM 
-  (SELECT * FROM t1 
+  (SELECT * FROM t1 FORCE INDEX(primary,idx)
      WHERE a BETWEEN 2 AND 7 OR pk=1000000) AS t;
 SELECT COUNT(*) FROM 
   (SELECT * FROM t1