Apparently there is a timing issue when using wait_for_ofs_sync:
```
[exception]: Executing test client: can't read "out_before": no such variable.
can't read "out_before": no such variable
```
The reason is that if the connection between the primary
and the replica is not established yet, the master_repl_offset
of the primary and replica in wait_for_ofs_sync is 0, and
the check fails, resulting in no replica client in the
client list below.
In this case, we need to make sure the replica is online
before proceeding.
Signed-off-by: Binbin <binloveplay1314@qq.com>
d9c41e9ef9