selftests/bpf: Amend test_tunnel to exercise BPF_F_TUNINFO_FLAGS

Get the tunnel flags in {ipv6}vxlan_get_tunnel_src and ensure they are
aligned with tunnel params set at {ipv6}vxlan_set_tunnel_dst.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220831144010.174110-2-shmulik.ladkani@gmail.com