selftests: drivers: dsa: add a subset of forwarding selftests

This adds an initial subset of forwarding selftests which I considered
to be relevant for DSA drivers, along with a forwarding.config that
makes it easier to run them (disables veth pair creation, makes sure MAC
addresses are unique and stable).

The intention is to request driver writers to run these selftests during
review and make sure that the tests pass, or at least that the problems
are known.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>