It’s occasionally possible for the cancellation of the service to happen
before connection_cb() gets scheduled in the other thread. The
locking/unlocking order of mutex_712570 requires:
 • test_threaded_712570(): lock mutex
 • test_threaded_712570(): start wait loop
 • connection_cb(): lock mutex
 • test_threaded_socket_service_finalize(): unlock mutex
 • test_threaded_712570(): end wait loop
 • test_threaded_712570(): unlock mutex

Fix that by quitting the main loop once connection_cb() has been called
(i.e. once the server thread has received the incoming connection
request), rather than just after the client thread (main thread) has
sent a connection request.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1679