selftests/bpf: Introduce verdict programs for sockmap_redir

Instead of piggybacking on test_sockmap_listen, introduce
test_sockmap_redir especially for sockmap redirection tests.

Suggested-by: Jiayuan Chen <mrpre@163.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20250515-selftests-sockmap-redir-v3-4-a1ea723f7e7e@rbox.co