nixos/test-driver: replace termlib with socat

telnetlib does not handle unicode, which is annoying when using systemctl.
Also this gives us a nice readline with history.

nixos/lib/test-driver/test-driver.py

@@ -462,9 +462,10 @@ class Machine:
         Should only be used during test development, not in the production test."""
         self.connect()
         self.log("Terminal is ready (there is no prompt):")
-        telnet = telnetlib.Telnet()
-        telnet.sock = self.shell  # type: ignore
-        telnet.interact()
+        subprocess.run(
+            ["socat", "READLINE", f"FD:{self.shell.fileno()}"],
+            pass_fds=[self.shell.fileno()],
+        )
 
     def succeed(self, *commands: str) -> str:
         """Execute each command and check that it succeeds."""