tests: drain the replayed serial backlog and stop the pty echo loop

With socket serial output now buffered and replayed on connect, a
late-connecting client receives the whole boot backlog. The pty
interaction test had three problems with that:

- pty_read() slept a second between 512-byte reads and the loop consumed
  one chunk per two-second tick, far too slow to drain the backlog. Read
  in larger chunks without the per-read sleep and drain everything
  available each round; bound the loop so a missing marker can't run to
  the harness timeout.

- it wrote the login keystrokes before reading, so the unread backlog
  back-pressured the sender and the keystrokes never reached the prompt.
  Start reading concurrently with typing instead.

- the socat pty was created with echo on, so the replayed backlog was
  echoed back to the guest as serial input, flooding it (UART input
  overrun, login never completing). Create the pty with echo=0.

Signed-off-by: Max Makarov <maxpain@linux.com>
Assisted-by: Claude:claude-opus-4-8 [Claude Code]

cloud-hypervisor/tests/integration.rs

@@ -2536,7 +2536,7 @@ mod common_parallel {
 
         let mut socat_command = Command::new("socat");
         let socat_args = [
-            &format!("pty,link={},raw", serial_socket_pty.display()),
+            &format!("pty,link={},raw,echo=0", serial_socket_pty.display()),
             &format!("UNIX-CONNECT:{}", serial_socket.display()),
         ];
         socat_command.args(socat_args);