gh-158451: Add _PyUnicodeWriter_FinishWithSize() test

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• Issue: C API: Rework and enhance the PyUnicodeWriter implementation #158451

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test_asyncio.test_sslproto failed with "env changed" on "Windows (free-threading) / Build and test (x64, switch-case)" CI. I just re-run the job, it's a flaky test.

test_shutdown_corrupted_ssl_sends_close_notify (test.test_asyncio.test_sslproto.ProactorStartTLSTests.test_shutdown_corrupted_ssl_sends_close_notify) ... Warning -- Uncaught thread exception: AssertionError
Exception in thread test-server:
Traceback (most recent call last):
  File "D:\a\cpython\cpython\Lib\test\test_asyncio\functional.py", line 255, in _run
    self._handle_client(conn)
    ~~~~~~~~~~~~~~~~~~~^^^^^^
  File "D:\a\cpython\cpython\Lib\test\test_asyncio\functional.py", line 264, in _handle_client
    self._prog(TestSocketWrapper(sock))
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\a\cpython\cpython\Lib\test\test_asyncio\test_sslproto.py", line 920, in server
    sock.unwrap()
    ~~~~~~~~~~~^^
  File "D:\a\cpython\cpython\Lib\ssl.py", line 1403, in unwrap
    s = self._sslobj.shutdown()
ConnectionAbortedError: [WinError 10053] An established connection was aborted by the software in your host machine

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "D:\a\cpython\cpython\Lib\threading.py", line 1218, in _bootstrap_inner
    self._context.run(self.run)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "D:\a\cpython\cpython\Lib\test\test_asyncio\functional.py", line 227, in run
    self._run()
    ~~~~~~~~~^^
  File "D:\a\cpython\cpython\Lib\test\test_asyncio\functional.py", line 261, in _run
    self._test._abort_socket_test(ex)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
  File "D:\a\cpython\cpython\Lib\test\test_asyncio\functional.py", line 110, in _abort_socket_test
    self.fail(ex)
    ~~~~~~~~~^^^^
  File "D:\a\cpython\cpython\Lib\unittest\case.py", line 772, in fail
    raise self.failureException(msg)
AssertionError: [WinError 10053] An established connection was aborted by the software in your host machine
ok