Integration Test: Race condition in test_aap2_close_on_send_to_sub

During the test test_aap2_close_on_send_to_sub, which checks that uD3TN closes the socket when it receives data unexpectedly in sub mode, the socket may be closed right before the second send call in AAP2Client.send, leading to errors in pipelines: https://gitlab.com/d3tn/ud3tn/-/jobs/6568803324

We should adapt send to only call socket.send once.