selftests: mlxsw: Add rtnetlink tests

Add a new test that is focused on rtnetlink configuration. Its purpose
is to test valid and invalid (as deemed by mlxsw) configurations and
make sure that they succeed / fail without producing a trace.

Some of the test cases are derived from recent fixes in order to make
sure that the fixed bugs are not introduced again.
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>