[DEFLAKE] Psync established after rdb load - beyond grace period

[roshkhatri]

Resolves: #2695

We saw the same error once in #2694 where the rdb took a little longer to load and thats where the test failed.

I ran it for 100 time in a workflow here https://github.com/roshkhatri/valkey/actions/runs/18605561661/job/53054114207 where I see max of 5.5 seconds to load the RDB.

Here I have increased the check for total of 6 seconds and IMO we would see it failing again, unless the rdb load get even slower in valgrid.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.65%. Comparing base (b4c93cc) to head (8d7ddb9).
⚠️ Report is 1 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2748      +/-   ##
============================================
+ Coverage     72.59%   72.65%   +0.05%     
============================================
  Files           128      128              
  Lines         71301    71300       -1     
============================================
+ Hits          51759    51800      +41     
+ Misses        19542    19500      -42     

see 17 files with indirect coverage changes

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