system-tests: Reduce flakiness of IPv6 prefix delegation

If one of the permutations failed, others running after
that would also fail because of leftover dhcpd running.
Make sure that dhcpd is killed if it's still running
since previous test.

Reported-at: https://bugzilla.redhat.com/2108726
Signed-off-by: Ales Musil <amusil@redhat.com>
Acked-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Numan Siddique <numans@ovn.org>
(cherry picked from commit b340d5d)

tests/system-ovn.at

@@ -5265,6 +5265,10 @@ ovn-nbctl set logical_router_port rp-public options:prefix=true
 ovn-nbctl set logical_router_port rp-sw0 options:prefix=true
 ovn-nbctl set logical_router_port rp-sw1 options:prefix=true
 
+OVN_POPULATE_ARP
+
+ovn-nbctl --wait=hv sync
+
 cat > /etc/dhcp/dhcpd.conf <<EOF
 option dhcp-rebinding-time 15;
 option dhcp-renewal-time 10;
@@ -5279,7 +5283,7 @@ chown root:dhcpd /var/lib/dhcp /var/lib/dhcp/dhcpd6.leases
 chmod 775 /var/lib/dhcp
 chmod 664 /var/lib/dhcp/dhcpd6.leases
 
-NS_CHECK_EXEC([server], [dhcpd -6 -f s1 > dhcpd.log &])
+NETNS_DAEMONIZE([server], [dhcpd -6 -f s1 > dhcpd.log 2>&1], [dhcpd.pid])
 ovn-nbctl --wait=hv sync
 
 OVS_WAIT_WHILE([test "$(ovn-nbctl get logical_router_port rp-public ipv6_prefix | cut -c4-15)" = ""])
@@ -5315,7 +5319,6 @@ OVS_WAIT_UNTIL([
     test "${total_pkts}" = "1"
 ])
 
-kill $(pidof dhcpd)
 kill $(pidof tcpdump)
 
 ovn-nbctl set logical_router_port rp-sw0 options:prefix=false