Fix replica online timing issue in failover test

[enjoy-binbin]

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.

[enjoy-binbin]

Ci link: https://github.com/valkey-io/valkey/actions/runs/10913083514/job/30288951861#step:8:5121

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.61%. Comparing base (ba71c7e) to head (afdf871). Report is 17 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1044      +/-   ##
============================================
- Coverage     70.62%   70.61%   -0.02%     
============================================
  Files           114      114              
  Lines         61672    61672              
============================================
- Hits          43555    43548       -7     
- Misses        18117    18124       +7     

see 9 files with indirect coverage changes