selftests: netfilter: switch to socat for tests using -q option

The nc cmd(nmap-ncat) that distributed with Fedora/Red Hat does not have
option -q. This make some tests failed with:

	nc: invalid option -- 'q'

Let's switch to socat which is far more dependable.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>