[tests] attempt to fix intermittent failures of MATN-12 (#6858)

The test failed because the script failed to read the output of ipaddr
rloc. This can be fixed by adding some wait time at the end of the
test.

tests/scripts/thread-cert/node.py

@@ -135,7 +135,7 @@ class OtbrDocker:
         assert launch_ok
 
         cmd = f'docker exec -i {self._docker_name} ot-ctl'
-        self.pexpect = pexpect.popen_spawn.PopenSpawn(cmd, timeout=10)
+        self.pexpect = pexpect.popen_spawn.PopenSpawn(cmd, timeout=30)
 
         # Add delay to ensure that the process is ready to receive commands.
         timeout = 0.4