selftests: nft_nat: ensuring the listening side is up before starting the client

The test_local_dnat_portonly() function initiates the client-side as
soon as it sets the listening side to the background. This could lead to
a race condition where the server may not be ready to listen. To ensure
that the server-side is up and running before initiating the
client-side, a delay is introduced to the test_local_dnat_portonly()
function.

Before the fix:
  # ./nft_nat.sh
  PASS: netns routing/connectivity: ns0-rthlYrBU can reach ns1-rthlYrBU and ns2-rthlYrBU
  PASS: ping to ns1-rthlYrBU was ip NATted to ns2-rthlYrBU
  PASS: ping to ns1-rthlYrBU OK after ip nat output chain flush
  PASS: ipv6 ping to ns1-rthlYrBU was ip6 NATted to ns2-rthlYrBU
  2023/02/27 04:11:03 socat[6055] E connect(5, AF=2 10.0.1.99:2000, 16): Connection refused
  ERROR: inet port rewrite

After the fix:
  # ./nft_nat.sh
  PASS: netns routing/connectivity: ns0-9sPJV6JJ can reach ns1-9sPJV6JJ and ns2-9sPJV6JJ
  PASS: ping to ns1-9sPJV6JJ was ip NATted to ns2-9sPJV6JJ
  PASS: ping to ns1-9sPJV6JJ OK after ip nat output chain flush
  PASS: ipv6 ping to ns1-9sPJV6JJ was ip6 NATted to ns2-9sPJV6JJ
  PASS: inet port rewrite without l3 address

Fixes: 282e5f8f ("netfilter: nat: really support inet nat without l3 address")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>