selftests: openvswitch: add a test for ipv4 forwarding

This is a simple ipv4 bidirectional connectivity test.

Signed-off-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Adrian Moreno <amorenoz@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>