Ci reported this failure:
```
[exception]: Executing test client: ERR FAILOVER target replica is not online..
ERR FAILOVER target replica is not online.
while executing
"$node_0 failover to $node_1_host $node_1_port"
```
We can see somehow the replica is not online in time and
casuing this failure, added a verify_replica_online to make
sure the replica is online for the test.
Signed-off-by: Binbin <binloveplay1314@qq.com>