code: func WithHijackClient(wrap func(tikv.Client) tikv.Client) MockTiKVStoreOption {
  	return func(c *mockOptions) {
  		c.clientHijack = wrap
  	}
  }
docs: // WithHijackClient hijacks KV client's behavior, makes it easy to simulate the network
  // problem between TiDB and TiKV.