selftests: net: set the exit code correctly in Python tests

selftests: net: set the exit code correctly in Python tests

Test cases need to exit with non-zero status if they failed,
we currently don't do that:

  # KTAP version 1
  # 1..3
  # # At /root/ksft-net-drv/drivers/net/./ping.py line 18:
  # # Check failed 1 != 2
  # not ok 1 ping.test_v4
  # ok 2 ping.test_v6
  # ok 3 ping.test_tcp
  # # Totals: pass:2 fail:1 xfail:0 xpass:0 skip:0 error:0
  ok 1 selftests: drivers/net: ping.py
  ^^^^

It's a bit tempting to make the exit part of ksft_run(),
but that only works well for very trivial setups. We can
revisit this later, if people forget to call ksft_exit().

Link: https://lore.kernel.org/r/20240417231146.2435572-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>