selftests: netfilter: Fix skip of wildcard interface test

selftests: netfilter: Fix skip of wildcard interface test

The script is supposed to skip wildcard interface testing if unsupported
by the host's nft tool. The failing check caused script abort due to
'set -e' though. Fix this by running the potentially failing nft command
inside the if-conditional pipe.

Fixes: 73db1b5dab6f ("selftests: netfilter: Torture nftables netdev hooks")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Link: https://patch.msgid.link/20250527094117.18589-1-phil@nwl.cc
Signed-off-by: Paolo Abeni <pabeni@redhat.com>