Fix test suite for duration formatting, not assuming locale

[avocade]

tests failed on my Swedish locale before, due to diff between eg 43,5 and 43.5. modified tests to explicitly set the locale for the formatter. now all tests pass as they should.

Fixes issue #113

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.94%. Comparing base (d40540a) to head (93cce0d).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #114   +/-   ##
=======================================
  Coverage   89.94%   89.94%           
=======================================
  Files          20       20           
  Lines        1929     1929           
  Branches       59       59           
=======================================
  Hits         1735     1735           
  Misses        135      135           
  Partials       59       59           

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