selftests: drv-net: add stdout to the command failed exception

ping prints all the info to stdout. To make debug easier capture
stdout in the Exception raised when command unexpectedly fails.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240416004556.1618804-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/lib/py/utils.py

@@ -33,7 +33,8 @@ class cmd:
         if self.proc.returncode != 0 and fail:
             if len(stderr) > 0 and stderr[-1] == "\n":
                 stderr = stderr[:-1]
-            raise Exception("Command failed: %s\n%s" % (self.proc.args, stderr))
+            raise Exception("Command failed: %s\nSTDOUT: %s\nSTDERR: %s" %
+                            (self.proc.args, stdout, stderr))
 
 
 def ip(args, json=None, ns=None):