bpo-30199: test_ssl closes all asyncore channels (#1381) (#1408)

bpo-30199: test_ssl closes all asyncore channels (#1381) (#1408)

AsyncoreEchoServer of test_ssl now calls
asyncore.close_all(ignore_all=True) to ensure that
asyncore.socket_map is cleared once the test completes, even if
ConnectionHandler was not correctly unregistered.

Fix the following warning:

Warning -- asyncore.socket_map was modified by test_ssl
  Before: {}
  After:  {6: <test.test_ssl.AsyncoreEchoServer.EchoServer.ConnectionHandler>}
(cherry picked from commit 1dae7450c68bad498e57800387b24cb103c461fa)