selftests: openvswitch: add drop reason testcase

Test if the correct drop reason is reported when OVS drops a packet due
to an explicit flow.

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