testing: Replace arp with ip neigh

The net-tools package, which provides arp and other older utilities like
ifconfig, is not installed anymore on Debian bullseye.

testing/tests/ikev2/dhcp-static-client-id/posttest.dat

@@ -7,5 +7,5 @@ venus::service isc-dhcp-server stop 2> /dev/null
 moon::iptables-restore < /etc/iptables.flush
 carol::iptables-restore < /etc/iptables.flush
 dave::iptables-restore < /etc/iptables.flush
-alice::arp -d 10.1.0.30
-alice::arp -d 10.1.0.40
+alice::ip neigh del 10.1.0.30 dev eth0
+alice::ip neigh del 10.1.0.40 dev eth0