…ythonGH-108370) (python#108407)

* In preauth tests of test_ssl, explicitly break reference cycles
  invoving SingleConnectionTestServerThread to make sure that the
  thread is deleted. Otherwise, the test marks the environment as
  altered because the threading module sees a "dangling thread"
  (SingleConnectionTestServerThread). This test leak was introduced
  by the test added for the fix of issue pythongh-108310.
* Use support.SHORT_TIMEOUT instead of hardcoded 1.0 or 2.0 seconds
  timeout.
* SingleConnectionTestServerThread.run() catchs TimeoutError
* Fix a race condition (missing synchronization) in
  test_preauth_data_to_tls_client(): the server now waits until the
  client connect() completed in call_after_accept().
* test_https_client_non_tls_response_ignored() calls server.join()
  explicitly.
* Replace "localhost" with server.listener.getsockname()[0].
(cherry picked from commit 592bacb)

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