bpf, testing: Refactor test_skb_segment() for testing skb_segment() on different skbs

Currently, test_skb_segment() builds a single test skb and runs
skb_segment() on it.

Extend test_skb_segment() so it processes an array of numerous
skb/feature pairs to test.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191025134223.2761-2-shmulik.ladkani@gmail.com