bpf: xfrm: Add selftest for bpf_xdp_get_xfrm_state()

This commit extends test_tunnel selftest to test the new XDP xfrm state
lookup kfunc.

Co-developed-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/e704e9a4332e3eac7b458e4bfdec8fcc6984cdb6.1702593901.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>