shelldriver: Redirect rx stderr to /dev/null

lrzsz also provides the `rx` command, and it will output the same banner
to stderr as the `lrz` command. Suppress stderr to handle this case

Signed-off-by: Joshua Watt <Joshua.Watt@garmin.com>

Author: JoshuaWatt

labgrid/driver/shelldriver.py

@@ -320,7 +320,8 @@ def _get_xmodem_rx_cmd(self, filename):
         if not self._xmodem_cached_rx_cmd:
             if self._run('which rx')[0]:
                 # busybox rx
-                self._xmodem_cached_rx_cmd = "rx '{filename}'"
+                # lrz may provide rx so redirect stderr for the same reason as below
+                self._xmodem_cached_rx_cmd = "rx '{filename}' 2>/dev/null"
             elif self._run('which lrz')[0]:
                 # redirect stderr to prevent lrz from printing "ready to receive
                 # $file", which will confuse the XMODEM instance