fix concurrent test failure

[aaronabramov]

i think this got copypasted from another test and was causing random failures when run concurrently (the shared directory was getting cleaned up).

i'll come up with a better solution that will automatically generate a test_dir name

[codecov-io]

Codecov Report

Merging #4159 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4159   +/-   ##
=======================================
  Coverage   60.55%   60.55%           
=======================================
  Files         196      196           
  Lines        6777     6777           
  Branches        6        6           
=======================================
  Hits         4104     4104           
  Misses       2670     2670           
  Partials        3        3

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[github-actions]

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