Fix test failures because of times with microseconds

[brianberlin]

connect #297

What

Makes use of new helper function that wraps Timex shift functions but also truncates microseconds precision.

Why

Timex.shift/2 adds microsecond precision to datetimes

Reference to issue in Timex: bitwalker/timex#731

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% 🎉

Comparison is base (988bd3d) 96.52% compared to head (4a56057) 96.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #299      +/-   ##
==========================================
+ Coverage   96.52%   96.55%   +0.02%     
==========================================
  Files          21       21              
  Lines         489      493       +4     
==========================================
+ Hits          472      476       +4     
  Misses         17       17              

Files Changed	Coverage Δ
lib/cocktail/builder/i_calendar.ex	97.14% <ø> (ø)
lib/cocktail/validation/interval.ex	100.00% <ø> (ø)
lib/cocktail/schedule_state.ex	100.00% <100.00%> (ø)
lib/cocktail/util.ex	100.00% <100.00%> (ø)
lib/cocktail/validation/day_of_month.ex	94.73% <100.00%> (ø)
lib/cocktail/validation/schedule_lock.ex	100.00% <100.00%> (ø)
lib/cocktail/validation/shift.ex	100.00% <100.00%> (ø)

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