fix(python): Fix a rounding error in parametric test datetimes generation

[stinodego]

I didn't account for the fact that Python floordiv rounds away from zero in negative numbers.

Also moving a constant that was only used in tests to a test fixture.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.93%. Comparing base (004efbe) to head (c21c7df).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16228   +/-   ##
=======================================
  Coverage   80.93%   80.93%           
=======================================
  Files        1394     1393    -1     
  Lines      179564   179568    +4     
  Branches     2908     2909    +1     
=======================================
+ Hits       145329   145335    +6     
+ Misses      33730    33728    -2     
  Partials      505      505           

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