Rule S101 [1] is not in the default settings. If you choose to enable it, you have the possibility of disabling it for your tests like so:

    [tool.ruff.lint.per-file-ignores]
    "tests/*" = ["S101"]

(Besides, this was about formatting, not linting, but I realize it's related.)

[1] https://docs.astral.sh/ruff/rules/