selftests/bpf: extend bpf tunnel test with gre

GRE is a commonly used protocol. Add GRE cases for both IPv4 and IPv6.

It also inserts different sized headers, which can expose some
unexpected edge cases.

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