selftests: nft_nat: Simplify port shadow notrack test

The second rule in prerouting chain was probably a leftover: The router
listens on veth0, so not tracking connections via that interface is
sufficient. Likewise, the rule in output chain can be limited to that
interface as well.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>