forwarding_test: Add a test for forwarding UDPv6

This is to test the UDPv6 forwarding path.

tests -
- Pass if no udp early demux v6 patch is present
- Fail if only 5425077d73e0c8e ("net: ipv6: Add early demux handler for UDP unicast")
- Pass if all existing early demux patches are present
	. 5425077d73e0c8e ("net: ipv6: Add early demux handler for UDP unicast")
	. 0bd84065b19bca1 ("net: ipv6: Fix UDP early demux lookup with udp_l3mdev_accept=0")
	. Ifa9c2ddfaa5b51 ("net: ipv6: reset daddr and dport in sk if connect() fails")

Change-Id: I12b9c1ca9c4045d0e330dc4f5d9eac7dcb179f8d