[DEFLAKE] Fix/cluster slot migration flaky test

[hanxizh9910]

Address: #2693

I add an additional check to ensure that all nodes recognize the full cluster (4 nodes) after a new node is added. This helps prevent the test from failing due to cluster gossip not being fully propagated.

I ran the test 100 times for both the unmodified and modified versions, and all runs passed:

Original (unmodified): CI run

Modified (with fix): CI run

For reference, this test previously failed once in https://github.com/valkey-io/valkey/actions/runs/18266295942/job/52001092528

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.46%. Comparing base (b4c93cc) to head (8df0ded).
⚠️ Report is 3 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2756      +/-   ##
============================================
- Coverage     72.59%   72.46%   -0.13%     
============================================
  Files           128      128              
  Lines         71301    71303       +2     
============================================
- Hits          51759    51671      -88     
- Misses        19542    19632      +90     

see 21 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[enjoy-binbin]

we have wait_for_cluster_size 4 in line 276, it is indedd checking the cluster_known_nodes

# Check if cluster size is consistent.
proc cluster_size_consistent {cluster_size} {
    for {set j 0} {$j < $cluster_size} {incr j} {
        if {[CI $j cluster_known_nodes] ne $cluster_size} {
            return 0
        }
    }
    return 1
}

[hanxizh9910]

You are right! i will remove this.