fix flaky INVALID_TOKEN server test

[marten-seemann]

Fixes #3219.

[codecov]

Codecov Report

Merging #3223 (adcb1ea) into master (2054732) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3223   +/-   ##
=======================================
  Coverage   85.43%   85.43%           
=======================================
  Files         133      133           
  Lines        9807     9807           
=======================================
  Hits         8378     8378           
  Misses       1057     1057           
  Partials      372      372           

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