selftests/bpf: Add verifier tests for bpf_sk_lookup context access

Exercise verifier access checks for bpf_sk_lookup context fields.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200717103536.397595-15-jakub@cloudflare.com

Author: jsitnicki