selftests/bpf: Prepare test_sock_addr for extension

test_sock_addr was not easy to extend since it was focused on sys_bind
and sys_connect quite a bit.

Reorganized it so that it'll be easier to cover new test-cases for
`BPF_PROG_TYPE_CGROUP_SOCK_ADDR`:

- decouple test-cases so that only one BPF prog is tested at a time;

- check programmatically that local IP:port for sys_bind, source IP and
  destination IP:port for sys_connect are rewritten property by tested
  BPF programs.

The output of new version:
  # test_sock_addr.sh 2>/dev/null
  Wait for testing IPv4/IPv6 to become available ... OK
  Test case: bind4: load prog with wrong expected attach type .. [PASS]
  Test case: bind4: attach prog with wrong attach type .. [PASS]
  Test case: bind4: rewrite IP & TCP port in .. [PASS]
  Test case: bind4: rewrite IP & UDP port in .. [PASS]
  Test case: bind6: load prog with wrong expected attach type .. [PASS]
  Test case: bind6: attach prog with wrong attach type .. [PASS]
  Test case: bind6: rewrite IP & TCP port in .. [PASS]
  Test case: bind6: rewrite IP & UDP port in .. [PASS]
  Test case: connect4: load prog with wrong expected attach type .. [PASS]
  Test case: connect4: attach prog with wrong attach type .. [PASS]
  Test case: connect4: rewrite IP & TCP port .. [PASS]
  Test case: connect4: rewrite IP & UDP port .. [PASS]
  Test case: connect6: load prog with wrong expected attach type .. [PASS]
  Test case: connect6: attach prog with wrong attach type .. [PASS]
  Test case: connect6: rewrite IP & TCP port .. [PASS]
  Test case: connect6: rewrite IP & UDP port .. [PASS]
  Summary: 16 PASSED, 0 FAILED

(stderr contains errors from libbpf when testing load/attach with
invalid arguments)
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>