[3.12] gh-108303: Create Lib/test/test_dataclasses/ directory (GH-108978) (#109674)

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

Move test_dataclasses.py and its "dataclass_*.py" modules into the
new Lib/test/test_dataclasses/ subdirectory.
(cherry picked from commit 14d6e197cc56e5256d501839a4e66e3864ab15f0)

Co-authored-by: Victor Stinner <vstinner@python.org>

* Fix Lint job: update Lib/test/.ruff.toml

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Co-authored-by: Victor Stinner <vstinner@python.org>