selftests: drv-net: gro: improve feature config

selftests: drv-net: gro: improve feature config

We'll need to do a lot more feature handling to test HW-GRO and LRO.
Clean up the feature handling for SW GRO a bit to let the next commit
focus on the new test cases, only.

Make sure HW GRO-like features are not enabled for the SW tests.
Be more careful about changing features as "nothing changed"
situations may result in non-zero error code from ethtool.

Don't disable TSO on the local interface (receiver) when running over
netdevsim, we just want GSO to break up the segments on the sender.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260113000740.255360-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>