Add basic tests checking working IPv46 forwarding.
All tests are executed in a per-test VNET jail, with routing-on-a-stick setup.
### Tests
#### IPv4
"Slow" path is enforced by using //Stream ID// ip option.
* fwd_ip_icmp_iface_fast_success - fastforwarding, routing to the interface, redirects disabled
* fwd_ip_icmp_gw_fast_success - fastforwarding, routing to the gateway, redirects disabled
* fwd_ip_icmp_iface_slow_success - slow forwarding, routing to the interface, redirects disabled
* fwd_ip_icmp_gw_slow_success - slow forwarding, routing to the gateway, redirects disabled
#### IPv6
"Slow" path is enforced by not disabling redirects.
* fwd_ip6_gu_icmp_iface_fast_success - fastforwarding, routing to interface, redirects disabled
* fwd_ip6_gu_icmp_gw_gu_fast_success - fastforwarding, routing to global unicast gateway, redirects disabled
* fwd_ip6_gu_icmp_gw_ll_fast_success - fastforwarding, routing to link-local gateway, redirects disabled
* fwd_ip6_gu_icmp_iface_slow_success - forwarding, routing to interface, redirects enabled
* fwd_ip6_gu_icmp_gw_gu_slow_success - forwarding, routing to global unicast gateway, redirects enabled
* fwd_ip6_gu_icmp_gw_ll_slow_success - forwarding, routing to global unicast gateway, redirects enabled