bpo-35998: Fix test_asyncio.test_start_tls_server_1() (GH-16815)

main() is now responsible to send the ANSWER, rather than
ServerProto. main() now waits until it got the HELLO before sending
the ANSWER over the new transport.

Previously, there was a race condition between main() replacing the
protocol and the protocol sending the ANSWER once it gets the HELLO.

TLSv1.3 was disabled for the test: reenable it.