selftests: openvswitch: add explicit drop testcase

Test explicit drops generate the right drop reason. Also, verify that
the kernel rejects flows with actions following an explicit drop.

Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>