test: make return types more precise

[bdrung]

The argument types should be as generic as possible, but the return types should be as specific as possible.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.40%. Comparing base (52560e6) to head (0c1c9ab).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #333   +/-   ##
=======================================
  Coverage   82.40%   82.40%           
=======================================
  Files          94       94           
  Lines       19334    19334           
  Branches     3113     3113           
=======================================
  Hits        15932    15932           
- Misses       2906     2907    +1     
+ Partials      496      495    -1     

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