This PR changes the testing infrastructure to run all black tests and:
* Pass if Ruff and Black generate the same formatting
* Fail and write a markdown snapshot that shows the input code, the differences between Black and Ruff, Ruffs output, and Blacks output
This is achieved by introducing a new `fixture` macro (open to better name suggestions) that "duplicates" the attributed test for every file that matches the specified glob pattern. Creating a new test for each file over having a test that iterates over all files has the advantage that you can run a single test, and that test failures indicate which case is failing.
The `fixture` macro also makes it straightforward to e.g. setup our own spec tests that test very specific formatting by creating a new folder and use insta to assert the formatted output.
I worked on #2993 and ran into issues that the formatter tests are failing on Windows because `writeln!` emits `\n` as line terminator on all platforms, but `git` on Windows converted the line endings in the snapshots to `\r\n`.
I then tried to replicate the issue on my Windows machine and was surprised that all linter snapshot tests are failing on my machine. I figured out after some time that it is due to my global git config keeping the input line endings rather than converting to `\r\n`.
Luckily, I've been made aware of #2033 which introduced an "override" for the `assert_yaml_snapshot` macro that normalizes new lines, by splitting the formatted string using the platform-specific newline character. This is a clever approach and gives nice diffs for multiline fixes but makes assumptions about the setup contributors use and requires special care whenever we use line endings inside of tests.
I recommend that we remove the special new line handling and use `.gitattributes` to enforce the use of `LF` on all platforms [guide](https://docs.github.com/en/get-started/getting-started-with-git/configuring-git-to-handle-line-endings). This gives us platform agnostic tests without having to worry about line endings in our tests or different git configurations.
## Note
It may be necessary for Windows contributors to run the following command to update the line endings of their files
```bash
git rm --cached -r .
git reset --hard
```
When creating a dict with string keys, some prefer to call dict instead of writing a dict literal.
For example: `dict(a=1, b=2, c=3)` instead of `{"a": 1, "b": 2, "c": 3}`.
