selftests: net: remove old setup_* scripts

selftests: net: remove old setup_* scripts

gro.sh and toeplitz.sh used to source in one of two setup scripts
depending on whether the test was expected to be run against
veth or a real device. veth testing is replaced by netdevsim
and existing "remote endpoint" support in our Python tests.
Add a script which sets up loopback mode.

The usage is a little bit more complicated than running
the scripts used to be. Testing used to work like this:

  ./../gro.sh -i eth0 ...

now the "setup script" has to be run explicitly:

  NETIF=eth0 ./../ksft_setup_loopback.sh ./../gro.sh

But the functionality itself is retained.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20251120021024.2944527-13-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>