Stabilize multi-raft test in CI

[akiradeveloper]

The node in Github Actions is too weak.

In such environment, the scheduling becomes unstable especially when many tasks are in Tokio queue. In this case, add-server requests could result in timeout due to being scheduled too late, for example.

Since the tests only want to check the logical aspect of the module, not the performance. I will set the scale as minimum.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.90%. Comparing base (c6be3c5) to head (2701967).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #457      +/-   ##
==========================================
+ Coverage   94.72%   94.90%   +0.17%     
==========================================
  Files          47       47              
  Lines        2446     2493      +47     
==========================================
+ Hits         2317     2366      +49     
+ Misses        129      127       -2     

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