samples/bpf: add multi-prog cgroup test case

create 5 cgroups, attach 6 progs and check that progs are executed as:
cgrp1 (MULTI progs A, B) ->
   cgrp2 (OVERRIDE prog C) ->
     cgrp3 (MULTI prog D) ->
       cgrp4 (OVERRIDE prog E) ->
         cgrp5 (NONE prog F)
the event in cgrp5 triggers execution of F,D,A,B in that order.
if prog F is detached, the execution is E,D,A,B
if prog F and D are detached, the execution is E,A,B
if prog F, E and D are detached, the execution is C,A,B

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>