selftests/bpf: Add bpf_fib_lookup test

This patch tests the bpf_fib_lookup helper when looking up
a neigh in NUD_FAILED and NUD_STALE state. It also adds test
for the new BPF_FIB_LOOKUP_SKIP_NEIGH flag.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230217205515.3583372-2-martin.lau@linux.dev