Tell 'timeout' to use signal 9, so command dies immediately

We were seeing flakes in test 320, seemingly because the 'docker run'
command would sometimes send a delete request for the container it had
started, before dying.

We need the container to carry on running so we can check it does get
attached when the cluster reaches quorum. Sending signal 9 means the
command dies immediately and does not get a chance to delete the
container.

test/320_claim_3_test.sh

@@ -66,7 +66,7 @@ weave_on $HOST1 launch --ipalloc-range $UNIVERSE $HOST2
 # Start another container on host1. The starting should block, because host1 is
 # not able to establish the ring due to host2 being offline.
 CMD="proxy_start_container $HOST1 --name c4 -e WEAVE_CIDR=$C4/12"
-assert_raises "timeout 5 cat <( $CMD )" 124
+assert_raises "timeout --signal=9 5 cat <( $CMD )" 137
 
 # However, allocation for an external subnet should not block.
 assert_raises "proxy_start_container $HOST1 -e WEAVE_CIDR=10.3.0.1/12"