selftests: net: Enable legacy netfilter legacy options.

Some specified options rely on NETFILTER_XTABLES_LEGACY to be enabled.
IP_NF_TARGET_TTL for instance depends on IP_NF_MANGLE which in turn
depends on IP_NF_IPTABLES_LEGACY -> NETFILTER_XTABLES_LEGACY.

Enable relevant iptables config options explicitly, this is needed
to avoid breakage when symbols related to iptables-legacy
will depend on NETFILTER_LEGACY resp. IP_TABLES_LEGACY.

This also means that the classic tables (Kernel modules) will
not be enabled by default, so enable them too.

Signed-off-by: Florian Westphal <fw@strlen.de>
[bigeasy: Split out the config bits from the main patch]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>