`test_ssl_in_multiple_threads` is flaky due to `with self.subTest` with threads

[colesbury]

Bug report

It's not safe to call with self.subTest from multiple threads concurrently. (See https://github.com/python/cpython/issues/131677)

======================================================================
ERROR: test_ssl_in_multiple_threads (test.test_ssl.ThreadedTests.test_ssl_in_multiple_threads)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/buildbot/buildarea/pull_request.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/test/test_ssl.py", line 3036, in test_ssl_in_multiple_threads
    raise cm.exc_value
  File "/Users/buildbot/buildarea/pull_request.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/threading.py", line 1075, in _bootstrap_inner
    self._context.run(self.run)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/Users/buildbot/buildarea/pull_request.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/threading.py", line 1017, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/buildbot/buildarea/pull_request.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/test/test_ssl.py", line 2968, in test_echo
    with self.subTest(client=ssl.PROTOCOL_TLS_SERVER, server=ssl.PROTOCOL_TLS_CLIENT):
         ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/buildbot/buildarea/pull_request.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/contextlib.py", line 148, in __exit__
    next(self.gen)
    ~~~~^^^^^^^^^^
  File "/Users/buildbot/buildarea/pull_request.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/unittest/case.py", line 562, in subTest
    with self._outcome.testPartExecutor(self._subtest, subTest=True):
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/buildbot/buildarea/pull_request.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/contextlib.py", line 148, in __exit__
    next(self.gen)
    ~~~~^^^^^^^^^^
  File "/Users/buildbot/buildarea/pull_request.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/unittest/case.py", line 81, in testPartExecutor
    self.result.addSubTest(test_case.test_case, test_case, None)
                           ^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'test_case'

From https://buildbot.python.org/?#/builders/1367/builds/987/

cc @ZeroIntensity