selftests/bpf: add tests for bpf_tcp_check_syncookie and bpf_skc_lookup_tcp

Add tests which verify that the new helpers work for both IPv4 and
IPv6, by forcing SYN cookies to always on. Use a new network namespace
to avoid clobbering the global SYN cookie settings.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>