test(datetime): improve test coverage

[mbhrznr]

working towards #3713.
pushes code coverage of the datetime sub-module closer to 100%.

currently only misses:

• a single branch within dateTimeFormatter.format(), which is tagged as FIXME
• a single branch within dateTimeFormatter.partsToDate(), where the local the format doesn't contain a day and the timezone isn't utc

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.37%. Comparing base (945e8f8) to head (9d3c726).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4649      +/-   ##
==========================================
+ Coverage   91.02%   91.37%   +0.34%     
==========================================
  Files         478      478              
  Lines       37336    37342       +6     
  Branches     5298     5325      +27     
==========================================
+ Hits        33985    34121     +136     
+ Misses       3289     3160     -129     
+ Partials       62       61       -1     

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[iuioiua]

Awesome work! Thank you.