selftests/bpf: Add a BPF selftest for bpf_skb_change_tail()

selftests/bpf: Add a BPF selftest for bpf_skb_change_tail()

As requested by Daniel, we need to add a selftest to cover
bpf_skb_change_tail() cases in skb_verdict. Here we test trimming,
growing and error cases, and validate its expected return values and the
expected sizes of the payload.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241213034057.246437-3-xiyou.wangcong@gmail.com