selftests: netdevsim: Add test cases for devlink-trap policers

Add test cases for packet trap policer set / show commands as well as
for the binding of these policers to packet trap groups.

Both good and bad flows are tested for maximum coverage.

v2:
* Add test case with new 'fail_trap_policer_set' knob
* Add test case for partially modified trap group

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