MDEV-9946: main.xtradb_mrr fails sporadically

Make the testcase stable by adding FORCE INDEX

mysql-test/r/xtradb_mrr.result

@@ -311,10 +311,10 @@ concat('c-', 1000 + C.a, '-c'),
 'filler'
 from t1 A, t1 B, t1 C;
 explain
-select count(length(a) + length(filler)) from t2 where a>='a-1000-a' and a <'a-1001-a';
+select count(length(a) + length(filler)) from t2 force index (a) where a>='a-1000-a' and a <'a-1001-a';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	range	a	a	9	NULL	99	Using index condition; Rowid-ordered scan
-select count(length(a) + length(filler)) from t2 where a>='a-1000-a' and a <'a-1001-a';
+select count(length(a) + length(filler)) from t2 force index (a) where a>='a-1000-a' and a <'a-1001-a';
 count(length(a) + length(filler))
 100
 drop table t2;

mysql-test/t/xtradb_mrr.test

@@ -33,8 +33,8 @@ insert into t2 select
 from t1 A, t1 B, t1 C;
 
 explain
-select count(length(a) + length(filler)) from t2 where a>='a-1000-a' and a <'a-1001-a';
-select count(length(a) + length(filler)) from t2 where a>='a-1000-a' and a <'a-1001-a';
+select count(length(a) + length(filler)) from t2 force index (a) where a>='a-1000-a' and a <'a-1001-a';
+select count(length(a) + length(filler)) from t2 force index (a) where a>='a-1000-a' and a <'a-1001-a';
 drop table t2;
 
 # Try a very big rowid