[3.11] gh-108303: Create Lib/test/test_dataclasses/ directory (#108978) (#109675)

gh-108303: Create Lib/test/test_dataclasses/ directory (#108978)

Move test_dataclasses.py and its "dataclass_*.py" modules into the
new Lib/test/test_dataclasses/ subdirectory.

Backport to 3.11: update Lib/test/.ruff.toml.

(cherry picked from commit 14d6e197cc56e5256d501839a4e66e3864ab15f0)