selftests/bpf: Add tests for accessing ingress_ifindex in bpf_sk_lookup

A new field was added to the bpf_sk_lookup data that users can access.
Add tests that validate that the new ingress_ifindex field contains the
right data.

Signed-off-by: Mark Pashmfouroush <markpash@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211110111016.5670-3-markpash@cloudflare.com