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

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>