[release-19.0] CI: Address data races on memorytopo Conn.closed (#15365)

[vitess-bot]

Description

This is a backport of #15365

[vitess-bot]

Review Checklist

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General

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.43%. Comparing base (97fdf78) to head (f87bff7).
Report is 1 commits behind head on release-19.0.

Additional details and impacted files

@@              Coverage Diff              @@
##           release-19.0   #15371   +/-   ##
=============================================
  Coverage         67.42%   67.43%           
=============================================
  Files              1560     1560           
  Lines            192642   192642           
=============================================
+ Hits             129896   129901    +5     
+ Misses            62746    62741    -5     

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