Update testthat edition

[paleolimbot]

Fixes #158.

[codecov-commenter]

Codecov Report

Merging #179 (0e06c1c) into main (10cede8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #179   +/-   ##
=======================================
  Coverage   93.67%   93.67%           
=======================================
  Files          50       50           
  Lines        3541     3541           
=======================================
  Hits         3317     3317           
  Misses        224      224           

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