bpf, selftests: use :: 1 for localhost in tcp_server.py

Using localhost requires the host to have a /etc/hosts file with that
specific line in it. By default my dev box did not, they used
ip6-localhost, so the test was failing. To fix remove the need for any
/etc/hosts and use ::1.

I could just add the line, but this seems easier.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/159594714197.21431.10113693935099326445.stgit@john-Precision-5820-Tower