tests: Fix MaxConnectionsPerIP setting in agreement/gossip tests

[algorandskiy]

Summary

#6171 decreased MaxConnectionsPerIP default value and it is not enough for agreement/gossip tests.
Here is a failure example for reference.

Test Plan

agreement/gossip tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.86%. Comparing base (a6123b6) to head (e7be338).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6173      +/-   ##
==========================================
+ Coverage   51.84%   51.86%   +0.02%     
==========================================
  Files         639      639              
  Lines       85488    85488              
==========================================
+ Hits        44319    44340      +21     
+ Misses      38360    38340      -20     
+ Partials     2809     2808       -1     

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