selftests: net: print report check location in python tests

Developing Python tests is a bit annoying because when test fails
we only print the fail message and no info about which exact check
led to it. Print the location (the first line of this example is new):

  # At /root/ksft-net-drv/./net/nl_netdev.py line 38:
  # Check failed 0 != 10
  not ok 3 nl_netdev.page_pool_check

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20240412141436.828666-4-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>