selftests: netfilter: nft_tproxy.sh: add tcp tests

The TPROXY functionality is widely used, however, there are only mptcp
selftests covering this feature.

The selftests represent the most common scenarios and can also be used
as selfdocumentation of the feature.

UDP and TCP testcases are split in different files because of the
different nature of the protocols, specially due to the challenges that
present to reliable test UDP due to the connectionless nature of the
protocol. UDP only covers the scenarios involving the prerouting hook.

The UDP tests are signfinicantly slower than the TCP ones, hence they
use a larger timeout, it takes 20 seconds to run the full UDP suite
on a 48 vCPU Intel(R) Xeon(R) CPU @2.60GHz.

Signed-off-by: Antonio Ojea <aojea@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>