MDEV-35124 Set innodb_snapshot_isolation=ON by default

From the very beginning, the default InnoDB transaction isolation level
REPEATABLE READ does not correspond to any well formed definition.
The main issue is the lack of write/write conflict detection.
To fix that and to make REPEATABLE READ correspond to Snapshot Isolation,
b8a6719889 introduced the Boolean
session variable innodb_snapshot_isolation. It was disabled by default
in order not to break any user applications.

In a new major version of MariaDB Server, we had better enable this
parameter by default.

mysql-test/suite/innodb/r/lock_isolation.result

@@ -11,6 +11,7 @@ a	b
 1	1
 2	2
 connect con_weird,localhost,root;
+SET innodb_snapshot_isolation=OFF;
 BEGIN;
 SELECT * FROM t;
 a	b