Fix flaky test 320

The intention, per the comments, is that host1 should claim from host2.
But host2 and host3 can carve up the space either way round.  If host1
claims from host3 then there is a race whether anyone finds out about
it.
Adding a 'prime' on host2 ensures the space is all owned by host2 at that point.

test/320_claim_3_test.sh

@@ -49,6 +49,7 @@ delete_persistence $HOST1 $HOST2
 # Now make host1 attempt to claim from host2, when host2 is stopped
 # the point being to check whether host1 will hang trying to talk to host2
 weave_on $HOST2 launch-router --ipalloc-range $UNIVERSE
+weave_on $HOST2 prime
 # Introduce host3 to remember the IPAM CRDT when we stop host2
 weave_on $HOST3 launch-router --ipalloc-range $UNIVERSE $HOST2
 weave_on $HOST3 prime