system-test: Fix tcpdump usage in LB template tests.

Otherwise tcpdump might not print anything until the interface goes away
making the test fail.  Also, make sure the capture is up before
continuing the test.  A similar fix was committed earlier via
6c4ffe5 ("system-test: Use OVS_WAIT_UNTIL for tcpdump start instead
fo sleep") but other tests were added in the meantime.

Suggested-by: Xavier Simonart <xsimonar@redhat.com>
Fixes: 086744a ("northd: Use LB port_str in northd.")
Reported-at: https://issues.redhat.com/browse/FDP-192
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
---
NOTE: we should probably try to find a unified way of using tcpdump
throughout the test suite.  However, since the LB template tests fail
quite often in CI, I'd like to find a quick fix for them for now so that
we simplify the work maintainers have to do on every patch.

tests/system-ovn.at

@@ -9213,8 +9213,9 @@ name: 'vport4' value: '999'
 NETNS_DAEMONIZE([vm1], [nc -k -l 42.42.42.2 4242], [nc-vm1.pid])
 
 NETNS_DAEMONIZE([vm1],
-    [tcpdump -n -i vm1 -nnleX -c6 udp and dst 42.42.42.2 and dst port 4343 > vm1.pcap 2>/dev/null],
+    [tcpdump -n -i vm1 -nnleX -c6 udp and dst 42.42.42.2 and dst port 4343 > vm1.pcap 2> vm1.pcap.stderr],
     [tcpdump1.pid])
+OVS_WAIT_UNTIL([grep "listening" vm1.pcap.stderr])
 
 # Make sure connecting to the VIP works (hairpin, via ls and via lr).
 NS_CHECK_EXEC([vm1], [nc 66.66.66.66 666 -z], [0], [ignore], [ignore])
@@ -9364,8 +9365,9 @@ name: 'vport4' value: '999'
 NETNS_DAEMONIZE([vm1], [nc -k -l 4242::2 4242], [nc-vm1.pid])
 
 NETNS_DAEMONIZE([vm1],
-    [tcpdump -n -i vm1 -nnleX -c6 udp and dst 4242::2 and dst port 4343 > vm1.pcap 2>/dev/null],
+    [tcpdump -n -i vm1 -nnleX -c6 udp and dst 4242::2 and dst port 4343 > vm1.pcap 2> vm1.pcap.stderr],
     [tcpdump1.pid])
+OVS_WAIT_UNTIL([grep "listening" vm1.pcap.stderr])
 
 # Make sure connecting to the VIP works (hairpin, via ls and via lr).
 NS_CHECK_EXEC([vm1], [nc 6666::1 666 -z], [0], [ignore], [ignore])