Backport streamlined NaiveDate unit tests

[Zomtir]

This patch set replaces tuples with an Option<NaiveDate> in some unit tests. This results in a less cluttered check function and keeps the code more in sync with the 0.5.x branch, where Result<NaiveDate, Error> will be more prevalent.

Initially suggested by @pitdicker in #1475 for checked_(add/sub)_days and extended on two more unit tests, which will likely be adapted in 0.5.x shortly.

Affected unit tests:

• checked_add_days
• checked_sub_days
• test_date_sub → test_date_signed_duration_since
• test_date_add → test_date_checked_add_signed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.80%. Comparing base (db1c8ef) to head (21abad7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1500   +/-   ##
=======================================
  Coverage   91.80%   91.80%           
=======================================
  Files          40       40           
  Lines       18329    18337    +8     
=======================================
+ Hits        16827    16835    +8     
  Misses       1502     1502           

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[pitdicker]

Nice 👍