PYTHON-5930 Move TestClientSSL to a shared, non-mirrored test file

Addresses review feedback: the tests in TestClientSSL are pure unit tests
(mocked or manual asyncio.run()) with no dependency on the async/sync test
class machinery, so they were needlessly duplicated by synchro. Moved them
to test/test_ssl_support.py, following the existing test_pyopenssl_context.py
precedent for standalone test files that aren't generated/mirrored.

Author: blink1073