selftests/bpf: Test BPF socket lookup and reuseport with connections

Cover the case when BPF socket lookup returns a socket that belongs to a
reuseport group, and the reuseport group contains connected UDP sockets.

Ensure that the presence of connected UDP sockets in reuseport group does
not affect the socket lookup result. Socket selected by reuseport should
always be used as result in such case.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Link: https://lore.kernel.org/bpf/20200722161720.940831-3-jakub@cloudflare.com