tests: Add missing reset_pcap_file() definition.

This is required to fix the test case "interconnection - static
multicast" after backporting the commit: b70e75f ("northd: Do not
relay local IP multicast (224.0.0.X)").

Reported-at: https://mail.openvswitch.org/pipermail/ovs-dev/2022-September/397558.html
Reported-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Han Zhou <hzhou@ovn.org>

tests/ovn.at

@@ -23070,6 +23070,17 @@ check_packets() {
     $at_diff exp rcv >/dev/null
 }
 
+reset_pcap_file() {
+    local iface=$1
+    local pcap_file=$2
+    check rm -f dummy-*.pcap
+    check ovs-vsctl -- set Interface $iface options:tx_pcap=dummy-tx.pcap \
+options:rxq_pcap=dummy-rx.pcap
+    check rm -f ${pcap_file}*.pcap
+    check ovs-vsctl -- set Interface $iface options:tx_pcap=${pcap_file}-tx.pcap \
+options:rxq_pcap=${pcap_file}-rx.pcap
+}
+
 ovn_init_ic_db
 ovn_start az1
 ovn_start az2