Update testing_boiler and unify test helpers

[FreezyLemon]

The unit tests are a bit messy at the moment. Duplicate code, subtly inconsistent behavior (test_case), bad naming, no documentation etc.

I tried to clean it up a bit. This also makes the problems mentioned in #108 easier to spot (ctrl-f "test_exact"). List of changes:

• Changed some function names:
  • try_create and create_case -> create_ok (dropped some verification but that really shouldn't matter[^1]. If it does, I can change this)
  • bad_create_case -> create_err (new: create_err returns the error, can be used for verification when more specific errors are returned)
  • test_case -> either test_exact or test_relative, depending on old behavior (some implementations used assert_eq, others used assert_relative_eq)
  • test_almost -> test_absolute
  • get_value -> create_and_get
• Added documentation to the test helpers
• Improve messages for test failures in common functions
• Rewrote some #[should_panic] tests to use test_none instead. Reads better IMHO

There are a few smaller things that could help further (re-enabling rustfmt for unit tests, dis-allowing clippy::excessive_precision), but those are less clear-cut, so I kept it to this changeset for now.

[^1]: The type of verification being done was extremely rudimentary, e.g. let dist = Dist::new(7.5, 3.5); and then assert_eq!(dist.a(), 7.5); plus assert_eq!(dist.b(), 3.5);.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.30%. Comparing base (aa276c8) to head (110fa86). Report is 37 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #269 +/- ## ========================================== - Coverage 90.62% 89.30% -1.33% ========================================== Files 50 50 Lines 11583 10976 -607 ========================================== - Hits 10497 9802 -695 - Misses 1086 1174 +88 ```

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[FreezyLemon]

Just noticed I missed some #[should_panic] tests that I still want to rewrite. And I guess I can also write some tests for the testing_boiler itself, now that it's being used for almost all unit tests

[FreezyLemon]

The changed lines have 100% coverage, but total coverage still went down because the PR removes circa 700 lines.

[YeungOnion]

I really appreciate this, I think it'll add a lot of value for contributors to have a consistent approach for tests.

I've no qualms with choices of names. I believe at this point, all arguments in new methods are stored in the struct. If that changes (sim to #198), then tests should be added with it.

I've reviewed the minimal functional changes to the boiler and I think it's fair to specify f64 until we actually implement over generic or multiple floats. These error messages will be more specific now as well, thanks 👍!

---

I believe that the only way this PR could allow behavior of the library to change in the future is if tests aren't the same, so my plan is to do some semantic diff and make sure everything in a pre-existing test module is a rename (or deletion, in the cases you footnote).

Sound reasonable?

[FreezyLemon]

Yeah, of course.

[FreezyLemon]

I think the auto-merge broke something here, master doesn't build atm.

[FreezyLemon]

See #277.