selftests: drv-net: add a Python version of the GRO test

selftests: drv-net: add a Python version of the GRO test

Rewrite the existing gro.sh test in Python. The conversion
not exact, the changes are related to integrating the test
with our "remote endpoint" paradigm. The test now reads
the IP addresses from the user config. It resolves the MAC
address (including running over Layer 3 networks).

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20251120021024.2944527-10-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>