selftests/bpf: expand bpf tunnel test to ipv6

The test only uses ipv4 so far, expand to ipv6.
This is mostly a boilerplate near copy of the ipv4 path.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>