selftests: net: py: teach cmd() how to print itself

Teach cmd() how to print itself, to make debug prints easier.
Example output (leading # due to ksft_pr()):

  # CMD: /root/ksft-net-drv/drivers/net/gro
  #   EXIT: 1
  #   STDOUT: ipv6 with ext header does coalesce:
  #   STDERR: Expected {200 }, Total 1 packets
  #           Received {100 [!=200]100 [!=0]}, Total 2 packets.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20260113000740.255360-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -41,7 +41,9 @@ class cmd:
         self.ret = None
         self.ksft_term_fd = None
 
+        self.host = host
         self.comm = comm
+
         if host:
             self.proc = host.cmd(comm)
         else:
@@ -99,6 +101,27 @@ class cmd:
             raise CmdExitFailure("Command failed: %s\nSTDOUT: %s\nSTDERR: %s" %
                                  (self.proc.args, stdout, stderr), self)
 
+    def __repr__(self):
+        def str_fmt(name, s):
+            name += ': '
+            return (name + s.strip().replace('\n', '\n' + ' ' * len(name)))
+
+        ret = "CMD"
+        if self.host:
+            ret += "[remote]"
+        if self.ret is None:
+            ret += f" (unterminated): {self.comm}\n"
+        elif self.ret == 0:
+            ret += f" (success): {self.comm}\n"
+        else:
+            ret += f": {self.comm}\n"
+            ret += f"  EXIT: {self.ret}\n"
+        if self.stdout:
+            ret += str_fmt("  STDOUT", self.stdout) + "\n"
+        if self.stderr:
+            ret += str_fmt("  STDERR", self.stderr) + "\n"
+        return ret.strip()
+
 
 class bkg(cmd):
     """