selftests: forwarding: README: Document customization

That any sort of customization is possible at all, let alone how it should
be done, is currently not at all clear. Document the whats and hows in
README.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Link: https://lore.kernel.org/r/e819623af6aaeea49e9dc36cecd95694fad73bb8.1711464583.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests: forwarding: README: Require diagrams

ASCII art diagrams are well suited for presenting the topology that a
test uses while being easy to embed directly in the test file iteslf.
They make the information very easy to grasp even for simple topologies,
and for more complex ones they are almost essential, as figuring out the
interconnects from the script itself proves to be difficult.

Therefore state the requirement for topology ASCII art in README.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

selftests: forwarding: Add initial testing framework

Add initial framework to test packet forwarding functionality. The tests
can run on actual devices using loop-backed cables or using veth pairs.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>