selftests: net: relocate gro and toeplitz tests to drivers/net

selftests: net: relocate gro and toeplitz tests to drivers/net

The GRO test can run on a real device or a veth.
The Toeplitz hash test can only run on a real device.
Move them from net/ to drivers/net/ and drivers/net/hw/ respectively.

There are two scripts which set up the environment for these tests
setup_loopback.sh and setup_veth.sh. Move those scripts to net/lib.
The paths to the setup files are a little ugly but they will be
deleted shortly.

toeplitz_client.sh is not a test in itself, but rather a helper
to send traffic, so add it to TEST_FILES rather than TEST_PROGS.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20251120021024.2944527-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>