Fix/improve tests

[drinkcat]

I was playing with jiff conversion, and noticed those tests that could be improved.

---

src/items: Fix timezone tests

The current tests look wrong ("%Z" shouldn't print an offset, but
this accidentally works due to the way the parsing logic operates),
fix that.

test: Replace %+ format with explicit one

%+ is an extension that jiff doesn't support (if we still want to
convert parse_datetime in the future, and since these are tests,
it's probably better to use an explicit format.

src/lib: Add end-to-end timezone test

Also make the offset test a little bit smarter.

[drinkcat]

I disagree with clippy here (1 * 3600 is better than 3600) -- changed the test to use +2 timezone.

[drinkcat]

Fuzzer fail is fixed by #145.

[cakebaker]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (cc7c143) to head (a967662).
Report is 4 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #146   +/-   ##
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