selftests/bpf: Add a basic fifo qdisc test

This selftest includes a bare minimum fifo qdisc, which simply enqueues
sk_buffs into the back of a bpf list and dequeues from the front of the
list.

Signed-off-by: Amery Hung <amery.hung@bytedance.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/20250409214606.2000194-9-ameryhung@gmail.com