test_runner: replace black with ruff format (#6268)

## Problem

`black` is slow sometimes, we can replace it with `ruff format` (a new
feature in 0.1.2 [0]), which produces pretty similar to black style [1].

On my local machine (MacBook M1 Pro 16GB):
```
# `black` on main
$ hyperfine "BLACK_CACHE_DIR=/dev/null poetry run black ."
Benchmark 1: BLACK_CACHE_DIR=/dev/null poetry run black .
  Time (mean ± σ):      3.131 s ±  0.090 s    [User: 5.194 s, System: 0.859 s]
  Range (min … max):    3.047 s …  3.354 s    10 runs
```
```
# `ruff format` on the current PR
$ hyperfine "RUFF_NO_CACHE=true poetry run ruff format"      
Benchmark 1: RUFF_NO_CACHE=true poetry run ruff format
  Time (mean ± σ):     300.7 ms ±  50.2 ms    [User: 259.5 ms, System: 76.1 ms]
  Range (min … max):   267.5 ms … 420.2 ms    10 runs
```

## Summary of changes
- Replace `black` with `ruff format` everywhere

- [0] https://docs.astral.sh/ruff/formatter/
- [1] https://docs.astral.sh/ruff/formatter/#black-compatibility

docs/sourcetree.md

@@ -129,13 +129,13 @@ Run `poetry shell` to activate the virtual environment.
 Alternatively, use `poetry run` to run a single command in the venv, e.g. `poetry run pytest`.
 
 ### Obligatory checks
-We force code formatting via `black`, `ruff`, and type hints via `mypy`.
+We force code formatting via `ruff`, and type hints via `mypy`.
 Run the following commands in the repository's root (next to `pyproject.toml`):
 
 ```bash
-poetry run black .  # All code is reformatted
-poetry run ruff .  # Python linter
-poetry run mypy .  # Ensure there are no typing errors
+poetry run ruff format . # All code is reformatted
+poetry run ruff check .  # Python linter
+poetry run mypy .        # Ensure there are no typing errors
 ```
 
 **WARNING**: do not run `mypy` from a directory other than the root of the repository.