selftests/bpf: use localhost in tcp_{server,client}.py

Bind and connect to localhost. There is no reason for this test to
use non-localhost interface. This lets us run this test in a network
namespace.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>