Add concurrent test cases

[PragmaTwice]

We need to add more cases about concurrency access from users, e.g. users can emit multiple migration tasks simultaneously from client.

[bseto]

I'm thinking of adding a SlotOnly: false test - in addition to the concurrency.

For the *redis.Client connection in Node ClusterNode, would you prefer it connect to a real kvrocks, or is using a redis-mock ok?

redis mock from here: https://github.com/go-redis/redismock

[git-hulk]

For the *redis.Client connection in Node ClusterNode, would you prefer it connect to a real kvrocks, or is using a redis-mock ok?

@bseto Yes, it would be nice to use Kvrocks, because some behaviors of Kvrocks is a bit different with Redis.