tests/echo_srv: make the main server logic resilient to ECONNABORTED from accept()

tests/echo_srv: make the main server logic resilient to ECONNABORTED from accept()

That should fix a race where the connect() directly followed by close()
in test_thread_echo_tcp_connect will cause the echo_srv to terminate
early, which results in connect() returning ECONNREFUSED in for other
threads.

This mainly happens on FreeBSD, but it can also happen on Linux.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>