Add a feature and method to override now

[pitdicker]

Updated version of #580. A comment in the related issue #105 (comment) has collected 28 thumb ups in three years. This may be our most popular feature request.

The only method added is Local::override_now(datetime: Option<DateTime<FixedOffset>>), and it affects Local::now() and Utc::now(). I made it a method on Local because it takes a datetime and an offset, while for Utc only a datetime would make sense.

The functionality is only available behind the test-override feature while running tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6f06e1a) 91.84% compared to head (481178e) 91.85%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1244   +/-   ##
=======================================
  Coverage   91.84%   91.85%           
=======================================
  Files          38       38           
  Lines       17518    17526    +8     
=======================================
+ Hits        16090    16098    +8     
  Misses       1428     1428           

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[pitdicker]

I just wondered: cfg(test) makes this method available in unit tests, but is it also available in integration tests? Yes.

[pitdicker]

Codecov correctly says the lines of this doctest are not covered. But I can't help it 😄.

[mgeisler]

Just a comment from a random bystander: instead of injecting this on a global level, people can opt-in code using a test double such as https://crates.io/crates/mockall_double. It does not "fix" existing code that uses chrono today, but it gives a more clean way to control the test setup.

[pitdicker]

I'm not going to push for this issue. Others can feel free to pick it up and/or advocate for it though.