fix: mock engine sharding (was implemented for cartridge only)

After implementing cartridge sharding mock tests were mostly useless: we didn't see how we modify SQL according to some specific shard.

Now (in a case of the filtered buckets when we send a query to exact shards) mock engine generates SQL for every shard. This behavior differs from the cartridge engine that groups buckets by replicasets first and then generated SQL for a replicaset bucket group. But the idea of the mock engine is to detect errors but not to implement actual Tarantool. So, lets make things simple and use this approach.