Fix race condition in the callback tests

[JamesWrigley]

Seen in https://github.com/JuliaParallel/DistributedNext.jl/actions/runs/23380060123/job/68018027988.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.65%. Comparing base (ff55fe6) to head (1e69d4f).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #57   +/-   ##
=======================================
  Coverage   88.65%   88.65%           
=======================================
  Files          12       12           
  Lines        2238     2238           
=======================================
  Hits         1984     1984           
  Misses        254      254           

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