selftests/bpf: Migrate *_POST_BIND test cases to prog_tests

Move all BPF_CGROUP_INET6_POST_BIND and BPF_CGROUP_INET4_POST_BIND test
cases to a new prog_test, prog_tests/sock_post_bind.c, except for
LOAD_REJECT test cases.

Signed-off-by: Jordan Rife <jrife@google.com>
Link: https://lore.kernel.org/r/20241022152913.574836-2-jrife@google.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>