Add tests to test_progs that exercise:

 - attaching/detaching/querying programs to BPF_SK_LOOKUP hook,
 - redirecting socket lookup to a socket selected by BPF program,
 - failing a socket lookup on BPF program's request,
 - error scenarios for selecting a socket from BPF program,
 - accessing BPF program context,
 - attaching and running multiple BPF programs.

Run log:

  bash-5.0# ./test_progs -n 70
  #70/1 query lookup prog:OK
  #70/2 TCP IPv4 redir port:OK
  #70/3 TCP IPv4 redir addr:OK
  #70/4 TCP IPv4 redir with reuseport:OK
  #70/5 TCP IPv4 redir skip reuseport:OK
  #70/6 TCP IPv6 redir port:OK
  #70/7 TCP IPv6 redir addr:OK
  #70/8 TCP IPv4->IPv6 redir port:OK
  #70/9 TCP IPv6 redir with reuseport:OK
  #70/10 TCP IPv6 redir skip reuseport:OK
  #70/11 UDP IPv4 redir port:OK
  #70/12 UDP IPv4 redir addr:OK
  #70/13 UDP IPv4 redir with reuseport:OK
  #70/14 UDP IPv4 redir skip reuseport:OK
  #70/15 UDP IPv6 redir port:OK
  #70/16 UDP IPv6 redir addr:OK
  #70/17 UDP IPv4->IPv6 redir port:OK
  #70/18 UDP IPv6 redir and reuseport:OK
  #70/19 UDP IPv6 redir skip reuseport:OK
  #70/20 TCP IPv4 drop on lookup:OK
  #70/21 TCP IPv6 drop on lookup:OK
  #70/22 UDP IPv4 drop on lookup:OK
  #70/23 UDP IPv6 drop on lookup:OK
  #70/24 TCP IPv4 drop on reuseport:OK
  #70/25 TCP IPv6 drop on reuseport:OK
  #70/26 UDP IPv4 drop on reuseport:OK
  #70/27 TCP IPv6 drop on reuseport:OK
  #70/28 sk_assign returns EEXIST:OK
  #70/29 sk_assign honors F_REPLACE:OK
  #70/30 sk_assign accepts NULL socket:OK
  #70/31 access ctx->sk:OK
  #70/32 narrow access to ctx v4:OK
  #70/33 narrow access to ctx v6:OK
  #70/34 sk_assign rejects TCP established:OK
  #70/35 sk_assign rejects UDP connected:OK
  #70/36 multi prog - pass, pass:OK
  #70/37 multi prog - drop, drop:OK
  #70/38 multi prog - pass, drop:OK
  #70/39 multi prog - drop, pass:OK
  #70/40 multi prog - pass, redir:OK
  #70/41 multi prog - redir, pass:OK
  #70/42 multi prog - drop, redir:OK
  #70/43 multi prog - redir, drop:OK
  #70/44 multi prog - redir, redir:OK
  #70 sk_lookup:OK
  Summary: 1/44 PASSED, 0 SKIPPED, 0 FAILED
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200717103536.397595-16-jakub@cloudflare.com