fix flaky session unit test

[marten-seemann]

Fixes #2531.

[codecov-io]

Codecov Report

Merging #2537 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2537      +/-   ##
==========================================
+ Coverage   86.12%   86.15%   +0.02%     
==========================================
  Files         122      122              
  Lines        9542     9542              
==========================================
+ Hits         8218     8220       +2     
+ Misses        985      984       -1     
+ Partials      339      338       -1     

Impacted Files	Coverage Δ
server.go	83.59% <0.00%> (+0.61%)	⬆️

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