selftests: mlxsw: RED: Test RED ECN nodrop offload

Extend RED testsuite to cover the new nodrop mode of RED-ECN. This test is
really similar to ECN test, diverging only in the last step, where UDP
traffic should go to backlog instead of being dropped. Thus extract a
common helper, ecn_test_common(), make do_ecn_test() into a relatively
simple wrapper, and add another one, do_ecn_nodrop_test().

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