selftests: nft_nat: Improve port shadow test stability

Setup phase in test_port_shadow() relied upon a race-condition:
Listening nc on port 1405 was started in background before attempting to
create the fake conntrack entry using the same source port. If listening
nc won, fake conntrack entry could not be created causing wrong
behaviour. Reorder nc calls to fix this and introduce a short delay
before testing the setup to wait for listening nc process startup.

Fixes: 465f15a6 ("selftests: nft_nat: add udp hole punch test case")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>