Fix the build_timestamps function for ranges starting at midnight

[hellais]

• Fix function
• Update tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.31%. Comparing base (a58b3d0) to head (8d86db4).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
- Coverage   84.31%   83.31%   -1.01%     
==========================================
  Files          76       76              
  Lines        4547     4554       +7     
==========================================
- Hits         3834     3794      -40     
- Misses        713      760      +47     

Flag	Coverage Δ
oonidata	77.86% <ø> (ø)
oonipipeline	87.34% <100.00%> (-1.77%)	⬇️

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