Fix the tcp_echo_server test to avoid hangs.

Shut down the writing end of the socket after writing, to allow the
server to end the stream, allowing a `read_to_end` on the reading end
to reach the end of the stream.

Author: sunfishcode

test-programs/artifacts/tests/tcp_echo_server.rs

@@ -106,6 +106,8 @@ fn tcp_echo_server() -> Result<()> {
     tcpstream.write_all(MESSAGE).context("write to socket")?;
     println!("wrote to echo server");
 
+    tcpstream.shutdown(Shutdown::Write)?;
+
     let mut readback = Vec::new();
     tcpstream
         .read_to_end(&mut readback)