Honor exact pytest nodes in the single-test workflow

[xiehuanyi]

The README's make test='FunctionalTests.test_background' test_one command passes a dotted unittest name to pytest, which exits with “file or directory not found.” Replacing it with a valid pytest node alone still collects the entire suite because tox prepends tests/ to every argument list.

Use tests/ as tox's default only when no positional arguments are supplied. Document the exact pytest node for the single-test target and the current uv sync development setup. The default full-suite target continues to select tests/.

Validation on Python 3.14.6/Linux: the original README selector exits 4; a valid exact node with the old prefix collects all 190 tests. The updated representative py314-locale-c-poller-poll tox invocation selects and passes exactly one requested test. Its full default run passes 187 tests with 3 skips. README rstcheck, Ruff lint/format and git diff --check pass. The other Python/platform/locale/backend combinations were not exercised; existing pytest warnings remain.

AI disclosure: OpenAI Codex assisted with investigation, implementation, command verification, and this description.

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